Surviving migrants are brought aboard Irish and Italian Navy life-boats in the area where their wooden boat capsized and sank off the coast of Libya, Aug 5, 2015. [Photo/Agencies]

ROME - A boat packed with up to 700 migrants capsized in the Mediterranean Sea off the coast of Libya on Wednesday and many were feared dead, officials and aid agencies said.

The Italian Coast Guard and the UN refugee agency UNHCR said about 400 people had been rescued and 25 bodies had been recovered so far, although it was still not clear exactly how many people were on board.

UNHCR spokesman Federico Fossi said as many as 100 people may have been in the hull when the steel boat capsized as rescue ships approached it. "Our worst fear was realized when the ship capsized before our very eyes," an Irish navy commander told state broadcaster RTE.

Overloaded boats carrying migrants often turn over due to sudden movements by the desperate passengers when they spot rescuers arriving.

The Mediterranean Sea has become the world's most deadly border area for migrants. More than 2,000 migrants and refugees have died so far this year in attempts to reach Europe by boat, compared with 3,279 deaths during the whole of last year, the International Organization for Migration said on Tuesday.

Wednesday's rescue operation, which included vessels from the Italian and Irish navies and the humanitarian group Medecins sans Frontieres, continued into the evening.

MSF, whose rescue ship Dignity 1 was in the area, said in a tweet that the boat was believed to have been carrying about 700 people. Irish Defence Minister Simon Coveney said he feared that "loss of life is likely to be significant".

In April, a fishing boat with up to 800 migrants sank, making it the deadliest shipwreck in the Mediterranean for decades and a symbol of Europe's long-running migrant crisis.

The 20-metre (66-foot) vessel capsized as it approached a merchant ship that had come to its assistance.