Geneva: As many as 500 people are feared dead after a shipwreck last week in the Mediterranean Sea, two international groups said today, describing survivors' accounts of panicked passengers who desperately tried to stay afloat by jumping between vessels.

The disaster happened in waters between Italy and Libya, based on accounts from 41 survivors who were rescued Saturday by a merchant ship, according to the UN refugee agency and the International Organization for Migration.

The tragedy ranks among the deadliest in recent years on the often-treacherous sea voyage along the central Mediterranean by refugees and migrants from Africa, the Middle East and beyond who have traveled in droves hoping to reach relatively peaceful and wealthy Europe.

While accounts provided by IOM and UNHCR varied slightly, both organizations said up to 200 people left the coastal town of Tobruk last week headed for a larger vessel already carrying hundreds of people in the Mediterranean.

IOM said the 200 people had left on several small boats, while UNHCR said 100 to 200 people left in a single 30-meter boat. The discrepancy in the accounts could not be immediately explained.

UNHCR said the larger boat was already facing "terribly overcrowded conditions" before the newcomers arrived. "Once transferred to the larger vessel now with an estimated 500 on board it began taking on water," IOM said, citing survivors' accounts. "The vessel started to sink and panicking passengers tried to jump into the smaller boats they had arrived in."

"The survivors told IOM that most of those aboard the larger vessel tragically died," the agency said in a statement.

It quoted an Ethiopian survivor it identified only as Mohamed as saying: "I saw my wife and my 2-month old child died at sea, together with my brother-in-law. ... The boat was going down ... Down. ... All the people died in a matter of minutes."

The survivors "drifted at sea for a few days, without food, without anything," Mohamed said, adding that he thought "I was going to die." He said the travelers had intended to go to Italy, not Greece.

In its statement, IOM Athens Chief of Mission Daniel Esdras called the accounts "heartbreaking" and said the organization was awaiting investigations by authorities "to better understand what actually happened and find hopefully evidence against criminal smugglers."

No national authorities in the area have reported any bodies washing ashore. Greek authorities said a cargo ship picked up 41 people on Saturday from a wooden boat that was without steering about 95 nautical miles south of the Greek mainland. The Greek authorities did not describe them as survivors or say anything about any boat sinking.