The boat was reported to have run into difficulty and sent out a distress call, but overturned as migrants rushed to one side.

According to the Irish navy,150 people were spotted in sea. The Doctors Without Borders (MSF) rescue vessel Dignity One (pictured above) was close to the area to offer first assistance. At least three other vessels, including the Irish vessel Le Niamh, were said to be heading to offer rescue.

The spokeswoman for UNHCR, Melissa Fleming, said some 250 people had been rescued.

"The boat overturned and sank quickly because it was made of metal," Federico Fossi, spokesman in Italy for the UNHCR told the AFP news agency. Fossi added that it was feared many people were stuck inside the ship when it sank.

The incident took place some 22 natical miles north of the Libyan city of Zuwarah.

Some 800 migrants drowned in a single incident off the Italian island of Lampedusa last April, triggering a major EU effort to improve rescue operations.

The EU has been looking for UN support to take action against the 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 fact the two governments are vying for power in the country.

A record of some 219,000 people crossed the Mediterranean in 2014, with Eritrea, Somalia and Syria among the countries from which most came. More than 100,000 have already made the perilous journey this year, while more than 2,000 have died.

rc/jil (AP, AFP, dpa)