Survivors from a fishing boat that sank off southern Greece in one of Europe's worst migrant disasters say up to 100 children may have been on board.
At least 78 people have already been confirmed dead in the disaster.
But scores more could still be missing at sea, with reports suggesting that up to 750 people were aboard the vessel.
The country's coastguard has been criticised for not intervening earlier but authorities say their offers of aid were refused.
Rescuers are still scouring the seas off Greece in a massive search operation, as hopes of finding more survivors dwindle.
Shocking accounts of a large number of women and children travelling in the hold of the ship have come via medics who treated the mostly male survivors.
Manolis Makaris, a doctor at Kalamata hospital, told local reporters that after lending his phone to a survivor to enable them to call home the man had spoken of a large number of children in the hold - possibly as many as 100.
Another reporter from Greece's ANT1 channel asked a survivor if there were 100 children on board, to which the survivor replied: "Yes".
The charity Save the Children, citing testimonies from survivors, also gave the same number.
The BBC has not been able to independently verify this figure.
But government spokesman Ilias Siakantaris said there were unconfirmed reports that up to 750 people in total were on the boat.
"We do not know what was in the hold... but we know that several smugglers lock people up to maintain control," he told the broadcaster ERT.
Activist Nawal Soufi was the first to raise the alarm after being contacted by people on the boat on Tuesday morning. She also believes around 750 people were on board.
The boat went down about 80km (50 miles) south-west of Pylos after 02:04 on Wednesday morning local time, according to the Greek coastguard.
A timeline provided by the coastguard said that initial contact was made with the fishing boat at 14:00 (11:00 GMT) the previous day, and no request for help had been made.
It said the Greek shipping ministry had made repeated contact with the boat and was told it simply wanted to sail on to Italy. A Maltese-flagged ship provided food and water at around 18:00, and another boat provided water three hours after that, it added.
Then at around 01:40 on Wednesday, someone on the boat is said to have notified the Greek coastguard that the vessel's engine had malfunctioned.
Shortly afterwards, the boat capsized, taking only 10 to 15 minutes to sink completely. A search and rescue operation was triggered but complicated by strong winds.
A coastguard spokesman told ERT that the boat's engine broke down in the early hours of Wednesday, then people on board started to move around causing it to capsize. He said everyone rescued was male.
Alarm Phone, an emergency helpline for migrants in trouble at sea, received the first call from the boat in distress just over an hour after the coastguard first made contact on Tuesday. In a Facebook post, Ms Soufi said the situation became "complicated" when a rescue boat approached the vessel and tied a rope to it while throwing bottles of water. She said some of the people on board then felt in "extreme danger" due to fears the rope could cause the boat to flip, and that fights on board over water could result in it capsizing. The boat then moved away. Ms Soufi said she stayed in touch with people on the vessel until 23:00 local time, saying she was reassuring them that the Greek coastguard would rescue them. On her final call, she said one man told her: "I feel like this is going to be our last night alive". She said that the people had "no intention of continuing" to Italy and "absolutely needed help in the waters". The conflicting accounts could be due to the coastguard speaking to the crew, while Ms Soufi and Alarm Phone were speaking to people on the vessel.
Alarm Phone complained that the coastguard was "aware of the ship being in distress for hours before any help was sent", adding that authorities "had been informed by different sources" that the boat was in trouble.
It added that people may have been scared to encounter Greek authorities because they were aware of the country's "horrible and systematic pushback practices".
The organisation later tweeted that questions should be aimed at European authorities.
"Ask them why there was insufficient rescue capacity although their distress had been known for so many hours. Ask why they prefer to blame those who died and survived, rather than responding to challenging questions about their failure to rescue."
Jérôme Tubiana of Médecins Sans Frontières told French radio that European and Greek authorities should have intervened earlier.
"It's really shocking to hear that Frontex flew over the boat and no-one intervened because the boat refused all offers of help... an overloaded boat is a boat in distress."
Italy's interior minister told SkyTG24 that the disaster happened in "Greece's search and rescue area, under the specific responsibility of that country".
Greece is observing three days of mourning over the disaster.
Campaigning has been suspended ahead of the parliamentary election on 25 June and a TV debate between the frontrunners due to happen on Thursday has been cancelled.
Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.
Last month the Greek government came under international criticism over video footage reportedly showing the forceful expulsion of migrants who were set adrift at sea.
At least 78 people have already been confirmed dead in the disaster.
But scores more could still be missing at sea, with reports suggesting that up to 750 people were aboard the vessel.
The country's coastguard has been criticised for not intervening earlier but authorities say their offers of aid were refused.
Rescuers are still scouring the seas off Greece in a massive search operation, as hopes of finding more survivors dwindle.
Shocking accounts of a large number of women and children travelling in the hold of the ship have come via medics who treated the mostly male survivors.
Manolis Makaris, a doctor at Kalamata hospital, told local reporters that after lending his phone to a survivor to enable them to call home the man had spoken of a large number of children in the hold - possibly as many as 100.
Another reporter from Greece's ANT1 channel asked a survivor if there were 100 children on board, to which the survivor replied: "Yes".
The charity Save the Children, citing testimonies from survivors, also gave the same number.
The BBC has not been able to independently verify this figure.
But government spokesman Ilias Siakantaris said there were unconfirmed reports that up to 750 people in total were on the boat.
"We do not know what was in the hold... but we know that several smugglers lock people up to maintain control," he told the broadcaster ERT.
Activist Nawal Soufi was the first to raise the alarm after being contacted by people on the boat on Tuesday morning. She also believes around 750 people were on board.
The boat went down about 80km (50 miles) south-west of Pylos after 02:04 on Wednesday morning local time, according to the Greek coastguard.
A timeline provided by the coastguard said that initial contact was made with the fishing boat at 14:00 (11:00 GMT) the previous day, and no request for help had been made.
It said the Greek shipping ministry had made repeated contact with the boat and was told it simply wanted to sail on to Italy. A Maltese-flagged ship provided food and water at around 18:00, and another boat provided water three hours after that, it added.
Then at around 01:40 on Wednesday, someone on the boat is said to have notified the Greek coastguard that the vessel's engine had malfunctioned.
Shortly afterwards, the boat capsized, taking only 10 to 15 minutes to sink completely. A search and rescue operation was triggered but complicated by strong winds.
A coastguard spokesman told ERT that the boat's engine broke down in the early hours of Wednesday, then people on board started to move around causing it to capsize. He said everyone rescued was male.
Alarm Phone, an emergency helpline for migrants in trouble at sea, received the first call from the boat in distress just over an hour after the coastguard first made contact on Tuesday. In a Facebook post, Ms Soufi said the situation became "complicated" when a rescue boat approached the vessel and tied a rope to it while throwing bottles of water. She said some of the people on board then felt in "extreme danger" due to fears the rope could cause the boat to flip, and that fights on board over water could result in it capsizing. The boat then moved away. Ms Soufi said she stayed in touch with people on the vessel until 23:00 local time, saying she was reassuring them that the Greek coastguard would rescue them. On her final call, she said one man told her: "I feel like this is going to be our last night alive". She said that the people had "no intention of continuing" to Italy and "absolutely needed help in the waters". The conflicting accounts could be due to the coastguard speaking to the crew, while Ms Soufi and Alarm Phone were speaking to people on the vessel.
Alarm Phone complained that the coastguard was "aware of the ship being in distress for hours before any help was sent", adding that authorities "had been informed by different sources" that the boat was in trouble.
It added that people may have been scared to encounter Greek authorities because they were aware of the country's "horrible and systematic pushback practices".
The organisation later tweeted that questions should be aimed at European authorities.
"Ask them why there was insufficient rescue capacity although their distress had been known for so many hours. Ask why they prefer to blame those who died and survived, rather than responding to challenging questions about their failure to rescue."
Jérôme Tubiana of Médecins Sans Frontières told French radio that European and Greek authorities should have intervened earlier.
"It's really shocking to hear that Frontex flew over the boat and no-one intervened because the boat refused all offers of help... an overloaded boat is a boat in distress."
Italy's interior minister told SkyTG24 that the disaster happened in "Greece's search and rescue area, under the specific responsibility of that country".
Greece is observing three days of mourning over the disaster.
Campaigning has been suspended ahead of the parliamentary election on 25 June and a TV debate between the frontrunners due to happen on Thursday has been cancelled.
Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.
Last month the Greek government came under international criticism over video footage reportedly showing the forceful expulsion of migrants who were set adrift at sea.
