Traffickers are accused of ramming a boat carrying more than 500 migrants from Egypt, causing it to sink in the Mediterranean off the coast of Malta, an intergovernmental agency reports.
The account is based on reports from two survivors who were rescued and taken ashore in Sicily after spending a day and a half in the water clinging to flotation devices.
The two Palestinian men, aged 27 and 33, were picked up by a Panamanian merchant ship. They told staff from the International Organization for Migration (IOM) that they had fled Gaza through Egypt and were requesting asylum.
They said that the migrants were forced to change boats several times, but resisted moving to a boat that they did not think was safe or big enough to carry them.
When they refused to cooperate, the traffickers, in a separate boat, reportedly rammed the boat the migrants were in, causing it to sink 300 miles southeast of Malta on September 10.
An IOM spokesperson said that if the reports were confirmed this was "not an accidental tragedy, but the apparent deliberate drowning of migrants by criminal gangs who extort money for their desperate journeys. Their actions are as callous as they are evil."
The Geneva-based IOM said it was investigating the account, which is difficult to verify since only nine others are thought to have survived.
If true, it would be the worst loss of life in the Mediterranean in decades.
On Sunday, another boat sank off the coast of Libya, and more than 200 African migrants are feared dead.
The IOM estimates that 3,000 people have died in the Mediterranean since January, far more than the 700 people who lost their life last year, and there has been a significant increase since June.
"The numbers dying off Europe's coasts are shocking and unacceptable," said IOM's director general William Lacy Swing in a statement.
"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," he added.
The survivors from the Malta shipwreck said they had left Egypt on September 6 with men, women and children on board. The passengers included Palestinians, Syrians, Egyptians, and Sudanese, they said.
The United Nations refugee agency reports that 130,000 people have arrived in Europe by sea already this year, more than double the numbers from last year. Italy has received more than 118,000 people, many of them rescued by the Italian navy's operation Mare Nostrum.
Angelina Jolie, the special envoy for the UNHCR, said: "There is a direct link between the conflicts in Syria and elsewhere and the rise in deaths at sea in the Mediterranean. Unless we address the root causes of these conflicts the numbers of refugees dying or unable to find protection will continue to rise."