Italian Prime Minister Matteo Renzi has led calls for more European Union action on sea migration after the latest deadly capsize of a boat in the Mediterranean.

Demanding a summit on the issue, Renzi said trafficking was "a plague in our continent" and bemoaned the lack of European solidarity.

The 20m (70ft) long boat was believed to be carrying up to 700 migrants, and only 28 survivors have been rescued.

Up to 1,500 migrants are now feared to have drowned this year alone.

Human smugglers are taking advantage of the political crisis in Libya to use it as a launching point for boats carrying migrants who are fleeing violence or economic hardship in Africa and the Middle East.

The UN High Commissioner for Refugees (UNHCR) said the latest sinking could amount to the largest loss of life during a migrant crossing to Europe.

'21st Century slavery'

Renzi singled out Libya as the key problem, saying it was the starting point for about 90% of the migrants reaching Italy by sea.

He said more rescue boats was not the issue, rather it was stopping the boats from departing.

Some Italian politicians had called for a naval blockade but Renzi said this would only help the smugglers as there would be more ships to rescue migrants.

Calling trafficking "the slavery of the 21st Century", he added: "It is unthinkable that in the face of such a tragedy, there isn't the feeling of solidarity which Europe has shown in other instances."

French President Francois Hollande called for "more boats, more over flights and a more intense battle against people trafficking", while Maltese PM Joseph Muscat said Europe and the international community would be judged by history if they continued to "turn a blind eye" to the plight of migrants.

Spanish PM Mariano Rajoy said: "This is the umpteenth time we hear of yet another human tragedy in the Mediterranean... Words won't do any more."

A meeting of EU ministers in Luxembourg on Monday has now added the issue to its agenda.

The EU has been criticised for its policy since the rescue operation, Mare Nostrum, was ended last year. Some EU members said they could not afford it and expressed concerns that it was encouraging more migrants.

It now runs a more limited border control operation called Triton.

A spokesperson for UN Secretary General Ban Ki-Moon said the deaths were an "urgent reminder of the critical need for a robust search and rescue capacity in the Mediterranean".

The international community had a duty to ensure the right to asylum for those fleeing war in search of safety, he added.

It is now dark in the area where the boat went down on Saturday night, 130 miles (210km) off the Italian island of Lampedusa and 17 miles from the Libyan coast. So far only 24 bodies have been retrieved.

The migrants reportedly fell overboard when they rushed to draw the attention of the passing Portuguese merchant ship King Jacob, causing their ship to capsize.

One survivor in the Cannizzaro hospital in Catania, Sicily, said there were as many as 950 people on board, although this has not been verified. He said many were locked below decks and not allowed to leave.

The UNHCR said that migrant boats had carried 13,500 people into Italian waters last week alone.

Last year, a record 170,000 people made the perilous crossing to Italy. Thousands died on the journey.