GENEVA (Reuters) - Some 320 migrants and refugees are now feared to have drowned off the Greek island of Crete last week as the deadliest toll on record so far in the Mediterranean keeps climbing, the International Organization for Migration said on Tuesday.

The Greek coastguard said on Saturday that the migrants who survived had told authorities their boat set sail from Egypt with about 350 people. On Friday, Greek authorities said 340 people were rescued after their boat sank and nine bodies had been recovered about 75 nautical miles off southern Crete.

But survivors taken to the port of Augusta, Italy, who were interviewed by IOM staff, reported that the vessel was carrying nearly twice the presumed number of passengers.

“We learned from survivors in Italy, in Augusta, that 648 or 650 men, women and children were on that ship. We heard both numbers from different survivors who took pains to explain that the smugglers made a count twice a day before the departure,” IOM spokesman Joel Millman told a briefing.

“We now fear some 320 migrants and refugees remain missing based on testimony received from survivors,” IOM said.

At least 10 bodies have been found, Millman said. The exact number of survivors was not clear but appeared to be over 300.

Some 206,400 migrants and refugees have crossed the Mediterranean this year, according to IOM. So far 2,809 deaths have been recorded, against 1,838 during the period last year.

“We are at almost 1,000 more deaths through five months of this year than we were last year and last year was the deadliest we know on record,” Millman said.

Most victims were sub-Saharan Africans departing from North Africa, mainly from Libya but also increasingly from Egypt, he said. Only 376 of the deaths this year occurred on the eastern sea route to Greece from Turkey.

“You’ve now had since the start of 2014, when this phenomenon of rising numbers across the Mediterranean happened, 10,000 deaths. That threshold has been crossed just in the last few days which is extremely disturbing,” U.N. refugee agency spokesman Adrian Edwards said.

Survivors from the latest boat wreck, which occurred in territory that falls under Egypt’s search and rescue jurisdiction, were taken to Italy and Egypt.

“There’s also a slightly happier story,” Millman said, referring to an Egyptian migrant woman named Mona who survived.

“She was travelling with three children, she thought they had all died. It turns out one of her sons was rescued and went on a different ship to Egypt. The mother is in Italy, the son is in Egypt, and IOM was able to inform the mother on Monday morning that one of her children survived the trip and was safe in Egypt.”