As many as 700 people were feared dead after a fishing boat packed with migrants capsized off the Libyan coast overnight in what officials said may be the Mediterranean's worst disaster as thousands flee poverty and war to Europe.

The United Nations High Commission for Refugees (UNHCR) said that around 50 people had been rescued of the 700 reported to be aboard.

Maltese Prime Minister Joseph Muscat said the rescue operation was continuing.

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

Italian Prime Minister Matteo Renzi called for an emergency meeting of European Union leaders this week, saying "We cannot remain insensitive when every day there is a massacre in the Mediterranean."

The incident happened in an area just off Libyan waters, 193km south of Lampedusa island, according to a report in the Times of Malta's website.

This comes amid a wave of migrants trying to leave Libya for Italian shores.

Al Jazeera's Paul Brennan, reporting from Catania in Sicily, off Italy's southern coast, said while the weather was sunny and good on Saturday, it later turned overcast and cloudy, which meant that the seas were choppy.

He said reports indicated that the approach of Portuguese ship prompted people on board the boat to shift to one side, upsetting the weight distribution and causing it to sink.

What's fuelling the migrant exodus from North Africa?

Pope Francis, who has spoken out repeatedly on the migrant crisis, repeated his call for quick and decisive action from the international community.

"They are men and women like us, our brothers seeking a better life, starving, persecuted, wounded, exploited, victims of war. They were looking for a better life, they were looking for happiness," he told tens of thousands of people in St. Peter's Square for his Sunday noon address.

More and more migrants anxious to escape conflict in North Africa and the Middle East are making the dangerous and often deadly crossing into European territory.

Around 13,000 migrants have been rescued at sea over the past week as good weather improved sailing conditions and spurred more boats to try to cross the Mediterranean Sea.

French President Francois Hollande said the EU had to do more, telling Canal+ television that rescue and disaster prevention efforts needed "more boats, more over flights and a much more intense battle against people trafficking."

Since the begining of the year at least 900 have died trying to reach Europe.