Up to 100 children may have been among about 500 migrants who drowned in the Mediterranean when their boat was sunk by people smugglers, an intergovernmental group has said after further interviews with survivors.

The International Organisation for Migration (IOM), which on Monday released the first information about the apparent mass murder during the voyage between Egypt and Malta – based on interviews with two Palestinian men on the vessel – said the other survivors corroborated the account and gave additional details. They reiterated the account that the traffickers become enraged when the migrants – most of whom were Palestinian, Egyptian, Sudanese or Libyan – refused to transfer to a small boat that appeared unseaworthy, and rammed the migrants' vessel with their craft. "After they hit our boat they waited to make sure it had sunk completely before leaving. They were laughing," one of the survivors told the IOM.

Based on other testimony about the traffickers counting 400-450 people who began the voyage aged 10 and older, and a total number aboard of about 500, the IOM said it was possible between 50 and 100 children had drowned.

A two-year-old girl was among nine migrants rescued, the IOM said. One of the nine has since died and another is in a critical condition, it added. The survivors include four Palestinian nationals, an Egyptian and a Syrian.

The voyage reportedly set off from Damietta in Egypt on 6 September, with the sinking taking place four days later. Two Palestinian survivors were pulled separately from the sea by a container ship and taken to Sicily.

Their testimony was released by the IOM on Monday, the same day that news emerged of about 200 migrants feared drowned after another boat sank off Libya.

One of the survivors of the incident – a 16-year-old Palestinian boy identified only as Hamed – gave an account to the Italian newspaper La Repubblica. "When they told us that we had to go on to that other boat, we refused because we'd doubtless have finished up at the bottom of the sea," he said. "At that point, the traffickers … rammed us, smashing the bows and we all finished up in the sea."

Hamed said he had been saved by the Panamanian-registered merchant vessel, Pegasus. He was quoted as saying that, after being rammed by the traffickers, "We were asking for help and about to drown, and they were watching us as if they were in a cinema."

He added: "A lot of us, myself included, did not know how to swim. I'd never seen the sea before. Seven or eight of us clung to a lifebelt, but as time went by a lot of them didn't make it and only two of us – I and another lad, a compatriot who was wearing a life jacket – were left. Then he disappeared too. Others were clinging to little bits of wood and the current carried them away. For many hours – I don't know how many – we remained in the water in those conditions.On Tuesday, the IMO said it had spoken to four of the surviving passengers, who have since been flown to Crete. They described being ordered by the traffickers to change boats three times during the first four days of the trip. "When they refused a fourth switch – because they felt the smaller vessel was unsafe – a violent argument ensued," the IOM said.

"The smugglers threatened that if the passengers did not board the smaller boat they would be returned to Egypt. The migrants persisted saying they would rather return than board the smaller boat."

At this stage, the survivors said, the 10 traffickers, said to be Palestinian and Egyptian, "began yelling and throwing sticks at the migrants".

"The smugglers' vessel approached the boat with migrants, some of whom managed to jump into the smaller boat. Witnesses say the smugglers forced them in the water and then rammed the bigger boat. It began to sink immediately while the smugglers stayed in the area until they were certain that the migrant's vessel had sunk," the IOM said.

One survivor said one passenger opted to hang himself as the boat sank rather than drown.

The two Palestinian survivors taken to Crete said they had paid $2,000 (£1,230) to a "travel office" in Gaza, using money they had received to rebuild their damaged home. They were told to head to a meeting point in Egypt, where they were taken by bus to Damietta. When they reached the ship, there were already migrants on board.

The IOM said: "The captain did a headcount and, without including minors under 10 years old, counted 400-450 migrants. Based on this testimony, the IOM believes up to 100 children may have been aboard and are lost at sea."

An estimated 300 people were put below deck and 200 on the upper deck, the survivors said. Those on the lower level were trapped and drowned immediately when the ship was rammed.

Some of the migrants clung to buoyant objects for up to three days, but many drowned when a change in the weather brought strong winds and high waves.

According to the IOM's tally, the two latest sinkings mean about 2,900 migrants have died this year in the Mediterranean, compared with 700 in 2013.

Huge numbers of people are attempting to flee from Africa to Europe, with numbers rising sharply this year, in part due to the continued violent chaos in Libya and Syria. According to the agency that monitors the EU's external borders, more migrants are likely to risk the dangerous crossings this year than at the height of the Arab spring.