Some of the accounts came from people who alleged they had survived the incident before being forced by Libyan forces loyal to Moamer Kadhafi to board another vessel that reached the Italian island of Lampedusa over the weekend.

“We have a pattern of information indicating that a ship capsized in the immediate vicinity of the Libyan coast last week due, it seems, to the excessive number of people it was carrying,” IOM spokesman Jean-Philippe Chauzy told AFP.

“Some of those who were shipwrecked managed to swim back to the coast,” Chauzy added. “The witnesses talked of bodies washed up on the coast.”

The IOM was unable to give a death toll.

Media accounts of the incident quoting refugees in Lampedusa or a Catholic bishop in Libya have been emerging since Friday.

“The very worrying thing is that among the survivors there were some who were forced by armed pro-Kadhafi men on board another vessel that arrived this weekend in Lampedusa,” Chauzy said.

The IOM spokesman said the agency’s staff had gathered accounts from half a dozen people who said “the overloaded ship was transporting about 500 to 600 people,”

A Somali refugee in Lampedusa whose son was on the boat received a call from relatives telling her that he was one of the victims, the Italian news agency ANSA reported on Sunday

Italian officials told AFP they had no information about the incident.

Bishop Giovanni Martinelli was quoted by Corriere della Sera daily as saying that he had met some of the boat’s Somali passengers before their departure and had received news of the incident from a nun.

Some 150 refugees fleeing Libya are believed to have died last month after their boat capsized in stormy weather in the middle of the Mediterranean. Italian coast guards managed to rescue 53 survivors from the sea.

On Sunday all 528 passengers from a boat that ran aground as it was approaching the port of Lampedusa at night were rescued.