GENEVA (Reuters) - At least 365 migrants have drowned in the Mediterranean this week in six sinkings, bringing the death toll so far this year to 4,636, already 1,000 more than in all of 2015, the International Organization for Migration said on Friday.

Migrants are seen during rescue operation in the Mediterranea Sea October 20, 2016. Yara Nardi/Italian Red Cross press office/Handout via Reuters

Migrants, mainly from West Africa, are taking to the sea from Libya in flimsy rubber boats, trying to reach Italian islands and Europe, where they have little hope of being granted asylum, IOM spokesman Leonard Doyle said.

“This is really a calamity in plain sight,” Doyle told a news briefing. “We are seeing really tragic scenes of rubber rafts going under the seas in the middle of winter in the Mediterranean.”

The president of the Italian unit of aid group Doctors Without Borders (MSF) said on Thursday that six migrants died and up to 100 more were missing and feared dead after their rubber boat sank. Aid groups had already put the death toll at 240 for the three days ending on Wednesday.

“Overall we’re counting 1,000 more migrant deaths in the Mediterranean compared to the same period last year,” Doyle said.

“This of course is due to appalling weather, migrants assuming and paying in the hope and expectation that they will get a decent passage across the Mediterranean, coming down to the beach (in Libya) and being confronted with a rubber raft, and not having any option, sometimes physical restraints on them even going back.”

The latest drowning victims were mostly West Africans, presumed to be from Gambia, Ghana, Mali, Nigeria and Senegal, he said.

“The migrants who are passing are primarily, sadly for them, going to end up as rejected asylum seekers, the vast majority of them have their claims rejected. So they would possibly fall into the category of economic migrants,” he said.

“They are destined to be deported or they live underground.”

The route between Turkey and Greece has been “effectively sealed off” since a deal between the European Union and Turkey last March, Doyle said.

Several ships have carried out rescue operations in recent days, he said. “The EU policy has undoubtedly led to the savings of hundreds of thousands of lives.”

“But an unintended consequence is that as they rescue migrants and destroy the smugglers’ boats, they go to ever flimsier vessels. And the migrants seem to be increasingly desperate to get to Europe and are taking ever greater chances to do so.”

There have been some 343,589 sea arrivals in Europe so far this year against 728,926 last year at this point, according to IOM figures.