As many as 700 people – including children – are feared dead after a boat carrying migrants capsized off the Libyan coast.

The United Nations High Commissioner for Refugees (UNHCR) has described the incident as one of the worst disasters seen in the Mediterranean migrant crisis.

Italian coastguards have rescued 49 survivors as the operation continues for the disaster, which happened in an area just off Libyan waters, south of the southern Italian island of Lampedusa.

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Rescuers from Italy and Malta have seen scores of bodies in Libyan waters, said Maltese military spokesman David Cioffi.

Children were reported to be among the dead.

A migrant is helped from a coast guard boat in the Sicilian harbour of Palermo. Photo: ABC

If confirmed, the disaster would be one of the worst seen during the decades-long migrant crisis in the southern Mediterranean and would bring the total number of dead since the beginning of the year to more than 1,500.

“At the moment, we fear that this is a tragedy of really vast proportions,” Carlotta Sami, a spokeswoman for the UNHCR, told SkyTG24 television.

Malta’s prime minister Joseph Muscat said the country’s armed forces were helping with the operation.

“They are literally trying to find people alive among the dead floating in the water,” he said.

“This could possibly be the biggest tragedy to have ever taken place in the Mediterranean.

“Children, men, and women have died.”

An Italian official confirmed that navy vessels were involved in the operation, which was being coordinated by the Italian coast guard in Rome.

But he said details were still being evaluated.

The boat is believed to have capsized when migrants moved to one side of the overcrowded vessel when 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,” Ms Sami said.

“The boat capsized because people moved to one side when another boat approached that they hoped would rescue them.”

The latest disaster is likely to fuel calls for a stronger response from Europe after heavy criticism of the Triton border protection operation which replaced a more comprehensive Italian search-and-rescue operation dubbed Mare Nostrum last year.

The toppling of former leader Muammar Gaddafi in 2011 has left Libya a lawless state, giving 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) estimated.

That is fewer than in the first four months of last year but the number of deaths has risen almost nine-fold.

Italy phased out Mare Nostrum or “Our Sea” late last year, making way for the European Union border control mission, Triton, which has been criticised by humanitarian groups and Italian authorities as it has a much smaller budget and a narrower remit.

Breaking:~700 people feared dead after boat carrying migrants capsized off Libyan coast #UNHCR http://t.co/UNVqLjhnrY pic.twitter.com/Q1LZVdmc2r

— Marc Richardson (@MarcRichNews) April 19, 2015

700 feared dead as migrant boat capsizes off Libyan coast http://t.co/We2Kf4haMf pic.twitter.com/iLqNjPZMKy — Dr Barkat SOOMRO (@DrBarkatSoomro) April 19, 2015

Boat capsizes off Libyan coast, 700 migrants feared dead – http://t.co/gQcCrp5KVW pic.twitter.com/pKlgawJ6Rx

— newsaiya (@hhaiya50) April 19, 2015

Pope calls for EU leaders to act

Pope Francis on Sunday led calls for European Union leaders to act to stem the loss of life.

“These are men and women like us, brothers seeking a better life,” he said in his weekly address to the Roman Catholic faithful in St Peter’s square, urging leaders to “act decisively and quickly to stop these tragedies from recurring”.

Amnesty’s John Dalhuisen called Sunday’s accident a “man-made tragedy of appalling proportions”.

“These latest deaths at sea come as a shock, but not a surprise,” he said.

Amnesty is among numerous NGOs calling for the restoration of an Italian navy search-and-rescue operation known as Mare Nostrum which was suspended at the end of last year.

Italy scaled back the mission after failing to persuade its European partners to help meet its operating costs of $12.5 million per month, amid divisions over whether the mission was unintentionally encouraging migrants to attempt the crossing.

Mare Nostrum has been partially replaced by a much smaller EU-run operation called Triton which has a fraction of the assets and specialist manpower previously deployed by Italy. Italian prime minister Matteo Renzi was expected to make a statement on the tragedy later Sunday following an emergency meeting with ministers involved in managing the migrant crisis.

The migrants seeking to reach Europe with the help of people smugglers are generally fleeing conflict or persecution in places such as Eritrea, Afghanistan and Syria, or poverty and hunger in sub-Saharan Africa and South Asia.

Drownings off #Libyan Coast is fast becoming a rapid killer for #Africa‘s most vulnerable. 1150 dead within a week. pic.twitter.com/LKDOg1FrZm — Abshir Mohamed Ahmed (@abshiraxmad) April 19, 2015

– with agencies