At least 367 migrants have been rescued and 25 bodies recovered after a fishing boat carrying as many as 700 people capsized on Wednesday in the Mediterranean Sea north of Libya, the Italian Coast Guard and Irish navy says.

Coast guard Cmdr. Filippo Marini said the rescue operation was still ongoing, involving seven ships. Survivors have indicated that between 400 and 700 people were on board the boat when it capsized, he added.

The exact number of those aboard might never be known, but authorities hope to have a better idea after survivors are interviewed.

The Irish naval vessel L.E. Niamh was one of several ships requested by the Italian Coast Guard to speed to the rescue of the overturned boat shortly before noon, Capt. Donal Gallagher told The Associated Press by phone.

When the Le Niamh arrived near the fishing boat, Marini said the migrants on board shifted to one side and the boat capsized.

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

An Italian military helicopter dropped additional life-rafts into the sea, where 150 people were initially spotted, Gallagher said.

By evening, the Le Niamh had 367 people aboard, including 13 children, the Irish military said.

Since the water was warm, rescuers worked with hope of finding more survivors, even as dusk approached. The vessel was reported to be 110 kilometres northwest of Tripoli, Libya's capital.

Irish Defence Minister Simon Coveney said he feared that "loss of life is likely to be significant."

'A mass grave'

Also involved in the rescue were an Italian vessel and Destiny I, a boat operated by Doctors Without Borders, which was tweeting pictures of the rescue operation.

Non-governmental organizations often join in migrant sea rescue operations, which are co-ordinated by the Italian Coast Guard and are now under the umbrella of a European Union task force known as Triton. The distressed vessel was reported to be 110 kilometres northwest of Tripoli, Libya's capital.

"We said it in April and we say it again today," Doctors Without Borders tweeted. "A mass grave is being created in the Mediterranean Sea and European states are responsible."

Her name is Azeel. She was under water when her father Mohammed pulled her to safety. His face says it all. <a href="https://twitter.com/hashtag/MSFSea?src=hash">#MSFSea</a> <a href="http://t.co/i4Pi7aysY0">pic.twitter.com/i4Pi7aysY0</a> —@MSF_Sea

This precious one year old child, from <a href="https://twitter.com/hashtag/Palestine?src=hash">#Palestine</a>, was almost lost today. <a href="http://t.co/pmbwjc856g">pic.twitter.com/pmbwjc856g</a> —@MSF_Sea

Before Wednesday's incident, the International Organization for Migration said nearly 2,000 migrants are believed to have lost their lives at sea since the start of this year. But the exact toll isn't known.

In April, a smuggling boat crammed with an estimated 800 migrants overturned, also off Libya's coast, where smugglers operate. Only 28 survivors, including two alleged smugglers, were found.

Fleeing war, persecution and poverty, the migrants travel overland for weeks or months from sub-Saharan Africa, the Horn of Africa, the Middle East and parts of Asia to reach Libya. There they set sail in flimsy motorized rubber dinghies or rickety old fishing boats. When the boats have problems, someone aboard contacts the coast guard by satellite phone requesting rescue. Other boats in distress are spotted by Triton air surveillance.

Most of the migrants hope to find asylum, relatives or jobs, mainly in northern Europe.