ROME -- Italy's coast guard said about 1,800 migrants were rescued from seven overcrowded vessels Saturday, while five corpses were found on a large rubber boat carrying 212 others. The dead bodies were found on board at the time of the rescue, a coast-guard representative said Sunday. The cause of death was not yet known, she said.

The Mediterranean Sea has become the world's most deadly barrier for migrants and refugees, with 3,500 believed to have died at sea last year and almost 2,000 so far this year. Many are fleeing poverty and violence in the Middle East and Africa.

While there was no breakdown by nationality of those rescued Saturday, about one-quarter of arrivals this year have been Eritreans, followed by Nigerians, Somalis, Sudanese and Syrians, according to the United Nations refugee agency.

Italy has had about 90,000 sea migrant arrivals so far this year, after receiving 170,000 in 2014, the agency says. Many of the newcomers look to move swiftly to wealthier northern Europe, including France and the U.K.

Nightly attempts by large groups of the estimated 5,000 migrants in Calais, France, to force their way through the rail tunnel linking Britain and France have provoked public anger and severely disrupted the flow of goods between the two countries.

(Reporting by Steve Scherer; Editing by Digby Lidstone)