At least 40 migrants have died at sea and hundreds more have been rescued from an overcrowded smugglers' boat in the Mediterranean sea.

The Italian navy said more than 320 people had been saved, but because the rescue was still ongoing authorities said the victims were "still being counted."

RaiNews24 TV said the people who died were found in the hold of the boat north of Libya.

They are thought to have suffocated after inhaling fumes from fuel after the vessel took on water.

The captain of the navy ship leading the rescue, Massimo Tozzi, said his men found the dead "immersed in water, fuel and human excrement."

More than 2,000 migrants have died at sea so far this year while trying to reach Europe from Libya, where human traffickers are based - including around 200 off the coast of Libya less than two weeks ago.

War, persecution and poverty have led them to leave the Middle East, Africa and Asia.

The Geneva-based International Organisation for Migration has said numbers of migrants trying to reach Europe by sea is heading for a record figure this year.

There have been just under 135,000 migrants arriving in Greece via Turkey, and along with migrants landing in Spain and Malta, the agency says 237,000 people have made the crossing so far in 2015 - up from 219,000 for all of 2014.

On the Greek island of Kos, desperate refugees have been pleading for help after thousands of people fleeing instability or poverty arrived over the summer.

With shortages of food and water, they have been sheltering from heat in tents and in an emergency centre at a sports stadium.

A ferry, the Eleftherios Venizizelos, has docked at Kos in order to help relieve a crisis that many observers have described as "chaotic".

The latest incident comes less than two weeks after around 200 people died when the wooden boat they were in capsized in the Mediterranean. 

The Irish naval vessel LE Niamh brought around 360 migrants ashore in Palermo following that tragedy.

More than 2,000 people fleeing poverty and war have died attempting to cross the Mediterranean to Europe this year.