Witnesses: Traffickers ram boat, 500 die

• NEW: Two survivors say they'd paid $2,000 at a "travel" office in Gaza for their voyage to Italy


• Migrant shipwreck survivors: Vessel was deliberately sunk by human smugglers


• Survivors: Traffickers became enraged and rammed the boat after they disobeyed them


• The IOM believes as many as 100 children may have been on board the boat

(CNN) -- Witnesses say as many as 500 migrants died in the Mediterranean Sea when human traffickers intentionally sunk their boat, the International Organization for Migration said Tuesday.

The survivors -- both Palestinian men from Gaza who were rescued separately after days in the water clinging to flotation aids -- said they had packed into a boat in Egypt's port of Damietta in Egypt and left on September 6. They estimated at least 400 men and women, in addition to as many as 100 children, were on board.

Two survivors interviewed by the IOM and Italian police in Sicily said the traffickers became enraged after the migrants -- who had already switched to small boats three times during their four days at sea -- refused to move to a smaller vessel. The migrants said they'd rather go back to Egypt than continue on in an even smaller vessel that they believed was not seaworthy.

The Palestinians' account was corroborated by several other survivors who were interviewed on the Greek island of Crete, the IOM said.

They said the smugglers, who they said were Palestinians and Egyptians, had begun yelling and throwing sticks at the migrants after they refused to leave the boat before then ramming the vessel. Some migrants managed to jump into the smaller boat, while one passenger "killed himself in despair by hanging," a survivor said.

"After they hit our boat, they waited to make sure that it had sunk completely before leaving. They were laughing," he added.

IOM: Many children among dead

By Tuesday morning, authorities in Italy, Malta and Greece confirmed the rescue of 10 migrants from the lost vessel, the IOM said. Three bodies have also been found.

Those on the boat included Syrians, Palestinians, Egyptians and Sudanese, as well as Palestinians, the witnesses said.

The two Palestinian survivors explained that they'd paid $2,000 -- money they'd gotten after receiving grants to rebuild their homes -- at what they called a "travel" office in Gaza for their voyage to Italy, according to the IOM.

Four buses took them to the Egyptian port near Alexandria, where about 200 people got on the ship's first deck and the other 300 people on the bottom deck.

Survivors surmised that those below deck were unable to escape the sinking boat and drowned almost immediately. Some people managed to stay afloat for three days, only to be done in by strong winds and waves that moved through the area.

'"If survivors' reports are confirmed, this will be the worst shipwreck of migrants in years, not an accidental tragedy, but the apparent deliberate drowning of migrants by criminal gangs who extort money for their desperate journeys," said IOM spokesman Leonard Doyle.

"Their actions are as callous as they are evil."

Italian navy finds dozens of bodies in migrant boat

'Shocking and unacceptable'

According to the IOM, the latest reported sinking would take the death toll off Europe's shores this year to nearly 3,000. In 2013, the organization's Missing Migrants Project estimated the total for the year to be 700 deaths.

"The numbers dying off Europe's coasts are shocking and unacceptable," said IOM director general William Lacy Swing.

"These are women, children and men who only hope for a more dignified life. The risks they take reflect their desperation and we cannot keep abandoning them to their fate."

Authorities are also investigating reports that 200 more people are missing off Libya and that another 15 drowned off the coast of Egypt, the IOM said.

Each year, tens of thousands of people are rescued from the Mediterranean, according to the European Union border agency, Frontex. The United Nations has expressed dismay over the rising number of migrants dying at sea.

READ: U.S. Navy rescues 282 apparent migrants in Mediterranean

CNN's Greg Botelho, Nana Karikari-apau and Carol Jordan contributed to this report.