A huge rescue operation is underway in the Mediterranean after a boat carrying hundreds of migrants trying to make their way to Europe sank.

The vessel capsized just off the coast of Libya.

So far around 400 people have been pulled to safety - but it is feared up to 300 may have drowned.

367 migrants have been rescued by the Irish navy's LE Niamh which is now making its way to Palermo to drop them off.

The migrants were travelling in an overcrowded fishing boat which sent out a distress call after running into difficulties some 15 nautical miles off the coast.

The first on the scene was the LE Niamh which deployed rigid hull inflatable boats (rhibs) - one each side of the striken vessel.

But, the Irish navy said that despite taking care to follow standard operating procedure, the 20 metre (66ft) vessel capsized.

Other rescue ships from charity Medecins Sans Frontieres (MSF) and the Italian coastguard also went to the scene.

A statement from the Irish Navy said: "At 8am Irish time, the LÉ Niamh was tasked by the Italian Marine Rescue Co-Ordination Centre (MRCC) to come to the assistance of a fishing vessel 110 kilometres north-west of Tripoli with an estimated 600 persons on board."

Spokesman for the United Nations Commission for Refugees Federico Fossi said as many as 100 people may have been in the hull when the boat capsized.

Will Turner, on the MSF ship Phoenix, which is on its way to the scene, said he understands dozens or more have lost their lives.

He told Sky News that the boat had now sunk, adding: "We know that there's dozens of people who have died, but we do not know the exact number.

"(The operation) is still going on... this boat ... would have been horrendously overcrowded. They are built to safely take about 50 people.

"With that number, simply a simple shift in people or a freak wave can be enough to make them capsize."
It is feared it could be the worst loss of life in one incident since 800 migrants died when a boat capsized in April.

According to the Defence Forces, the boat was 68 miles (110km) northwest of Tripoli when the LE Niamh reached it.
It had set out from Zuwarah, close to the Tunisian border.

More than 2,000 people trying to make the crossing from Libya to Europe have died this year, according to the International Organisation for Migration.

Defence Minister Simon Coveney earlier confirmed that: “At 8 am Irish time, the LÉ Niamh was tasked by the Italian Marine Rescue Co-Ordination Centre (MRCC) to come to the assistance of a fishing vessel north-west of Tripoli, which was in distress with an estimated 600 persons on board.”

The LE Niamh worked alongside Italian naval vessels and an Médecins Sans Frontiéres (MSF) vessel, Dignity. The operation will see a number of smaller crafts, including inflatable boats and lifesrafts, deployed. 

The Niamh has a crew of 55 Naval Service personnel, and two medics from the Army Medical Corps.

It has rescued more than 600 migrants since its tour of duty began there last month.

The LE Niamh replaced the LE Eithne, which was deployed to the area two months earlier.