As many as 700 people were feared dead after a fishing boat packed with migrants capsized off the Libyan coast overnight, in what may be one of the worst disasters of the Mediterranean migrant crisis, officials said on Sunday.

Twenty eight people were rescued in the incident, which happened just off Libyan waters, south of the southern Italian islandof Lampedusa, Antonino Irato, a senior official from the Italian border police, told television station RaiNews24.

He said 24 bodies had been recovered.

If confirmed, the death toll would bring the total number of dead since the beginning of the year to more than 1,500.

The new deaths fuelled calls for a stronger response from Europe to the increasingly deadly migrant crisis playing out in theMediterranean. International aid groups and Italian authorities have criticised Europe's so-called "Triton" border protection operation, which recently replaced a more comprehensive Italian search-and-rescue mission.

"A tragedy is unfolding in the Mediterranean, and if the EU and the world continue to close their eyes, it will be judged in the harshest terms as it was judged in the past when it closed its eyes to genocides when the comfortable did nothing," Maltese Prime Minister Joseph Muscat said.

Italian officials said navy and coast guard vessels, as well as merchant ships in the area and a Maltese patrol boat, were involved in the search and rescue operation, which was being coordinated by the Italian coast guard in Rome.

"They are literally trying to find people alive among the dead floating in the water," he added.

There was still no decision on where the survivors and the bodies that had been recovered would be taken.

The boat is believed to have capsized when the migrants shifted to one side of the overcrowded vessel as a merchant ship approached.

"The first details came from one of the survivors who spoke English and who said that at least 700 people, if not more, were on board. The boat capsized because people moved to one side when another vessel that they hoped would rescue them approached," said Carlotta Sami, a spokeswoman for the United Nations High Commissioner for Refugee.

"SYSTEMATIC SLAUGHTER"

Italian Prime Minister Matteo Renzi said Europe was witnessing "systematic slaughter in the Mediterranean".

"How can we remain insensible when we're witnessing entire populations dying at a time when modern means of communications allow us to be aware of everything?" Renzi said at a political event in Mantua.

On his way back to Rome, where he was expected to give a news conference later, Renzi spoke by telephone to French President Francois Hollande.

The leader of the anti-immigrant Northern League party, Matteo Salvini, who has made migration one of the centrepieces of his political agenda, called for a an immediate naval blockade of the coast of Libya.

The lawless state of Libya, following the toppling of former leader Muammar Gaddafi in 2011, has left criminal gangs of migrant smugglers a free hand to send a stream of boats carrying desperate migrants from Africa and the Middle East.

Around 20,000 migrants have reached the Italian coast this year, the International Organisation for Migration (IOM) estimates. That is fewer than in the first four months of last year, but the number of deaths has risen almost nine-fold.

In 2013, the previous government initiated the search-and-rescue operation "Mare Nostrum" or "Our Sea" after hundreds drowned in an incident off the coast of Lampedusa. The operation was cancelled last year, because of the cost and because some politicians said it encouraged migrants to depart by raising their hopes of being rescued.

That made way for the European Union's border control mission, Triton. However Triton, which has a much smaller budget and narrower remit, has been criticised by humanitarian groups and Italy as inadequate to tackle the scale of the problem.