ATHENS • Mr Muaz Mahmud's wife and two-month-old son barely had time to scream as the cold waters of the Mediterranean surged over their heads.

Minutes earlier, they had been forced by smugglers to abandon the boat they had chartered in Libya last week for a larger boat already packed with 300 migrants heading towards Italy.

As the bigger boat took on more passengers, it began to list, Mr Mahmud said on Thursday.

When it capsized, hundreds of panic-stricken people - many of whom could not swim - were thrown into the sea.

Mr Mahmud was one of only 41 people who were eventually saved by a passing merchant ship and brought to Greece on April 16.

At a news conference in Athens, Mr Mahmud and another survivor of last week's shipwreck recounted grim details of the sinking, in which 500 passengers may have died.

The incident makes it the deadliest episode for asylum seekers trying to reach Europe since more than 800 drowned in April last year in a boat trying to reach Italy.

The authorities in the region have not yet verified the disaster.

But the latest tragedy, if confirmed, would bring the number of migrants who have died in the Mediterranean between North Africa and Europe this year to nearly 800, the International Organisation for Migration said.

Mr Mahmud, a 25-year-old from Ethiopia, said that he, his wife and their infant son had boarded a boat late last week in Tobruk, in eastern Libya, with around 200 other migrants, mostly from Somalia, Sudan, Eritrea and Ethiopia.

They paid the smugglers US$1,800 (S$2,400) per passenger to take them to Italy, where they hoped to apply for asylum.

After sailing for about 15 hours, the smugglers told the passengers in Mr Mahmud's boat that everyone would be transferred to a larger ship that was waiting in the middle of the sea.

As their boat approached the new ship, the passengers grew nervous when they realised the other vessel was already packed with people.

Mr Mahmud said that when he and others tried to resist, the smugglers forced them to disembark.

As he and his family were being transferred to the other ship, he recalled, it began to capsize.

Flung into the water, Mr Mahmud said he saw his wife and child, who were out of his reach, drown along with hundreds of others who were travelling with them.

Mr Mahmud, one of the few people aboard who could swim, said he made his way towards a boat that still had about 30 people on board.

Those on board threw him a rope, saving him and about 10 other people who had also scrambled to get to safety, he said.

"My wife and baby, they were dead," Mr Mahmud said.

"They drank the water of the ocean when the boat went down."

NEW YORK TIMES