ROME — Hundreds of people were feared dead on Sunday after a ship crowded with migrants capsized and sank in the Mediterranean, as the authorities described a grisly scene of bodies floating and submerging in the warm waters, with the majority of the dead apparently trapped in the ship at the bottom of the sea.

The fatal shipwreck may prove to be the Mediterranean’s deadliest migrant disaster ever and is only the latest tragedy in Europe’s migration crisis. Warmer spring weather has unleashed a torrent of smuggler boats, mostly from Libya, bearing migrants and refugees from the Middle East and Africa, often fleeing war and poverty for a foothold in Europe.

Death at sea has become a grimly common occurrence: Even before this weekend’s sinking, humanitarian groups estimated that 900 migrants had already died this year, compared with 90 during the same period a year ago. That figure could rise sharply, as officials estimate that 700 people may have drowned in the weekend disaster.

The rising death toll is renewing criticism of the European response, especially the Triton program, introduced in November to patrol the Mediterranean and rescue migrants. United Nations officials and humanitarian groups have argued that Triton is too limited in scope and resources and thus is placing migrants at grave risk.