At least 25 migrants were confirmed to have died Wednesday, and hundreds more are feared to have drowned, after a boat packed with around 700 people capsized in the Mediterranean off the coast of Libya, officials said.

The UN refugee agency UNHCR and the Italian coast guard said about 400 people had been rescued from the sea thus far, though it was still not clear precisely how many people were on the doomed ship.

The boat reportedly capsized as rescue ships approached it, with as many as 100 people in the ship's hull when it went over.

"Our worst fear was realized when the ship capsized before our very eyes," an Irish navy commander told Irish state broadcaster RTE.

Crowded migrant boats often capsize due to sudden movements made by desperate passengers when they see rescuers approaching.

Italian coast guard spokesman Filippo Marini said that while the sea was very calm, "the boat overturned and sank quickly because it was made of metal."

Perilous crossing

More than 2,000 migrants have died so far this year attempting to reach Europe by boat, compared with 3,270 deaths for the entirety of last year, according to figures released on Tuesday by the International Organization for Migration.

Rescue operations, which include Italian and Irish navy vessels and the medical charity group Doctors Without Borders, are ongoing.

Doctors Without Borders said that the boat was believed to have been carrying 700 people. Irish Defense Minister Simon Coveney, who confirmed that the navy vessel Le Niamh had been diverted to the scene, said he feared that "loss of life is likely to be significant."

"The crew of the Niamh are working flat out with their counterparts to rescue as many as possible," he said.

The deadliest shipwreck in the Mediterranean in decades occurred this past April, when a fishing boat with up to 800 migrants sank. It has since become a symbol for Europe's ongoing migrant crisis.

The EU has been looking for UN support to take action against migrant smugglers, who have profited from the turmoil within Libya, where most of the migrants begin their sea journey. Intervention has been made more difficult by the struggle for power between the country's two governments.

bw/cmk (Reuters, AFP, dpa)