As many as 400 migrants are believed to have drowned of the coast of Libya, after their boat capsized, according to reports from survivors.

This is the latest in a number of tragedies in which migrants have died crossing the Mediterranean, trying to reach Italy and other European countries.

The boat, carrying about 550 migrants in total, flipped about 24 hours after leaving the Libyan coast, according to some of the 150 survivors who were rescued and brought to a southern Italian port on Tuesday morning, Save the Children reported.

Before this incident, 500 people had died crossing the Mediterranean from Africa this year, up sharply from 47 in the same period of 2014, according to the Geneva-based International Organisation for Migration (IOM).

Last year up to 3,072 people died trying to make the crossing, more than double the previous year. Since 2000, an estimated 22,000 migrants have died trying to reach Europe.

In November last year Pope Francis raised the issue at the European Parliament, and said: "We cannot allow the Mediterranean to become a vast cemetery.

"The absence of mutual support within the European Union runs the risk of encouraging... solutions which fail to take into account the human dignity of immigrants, and thus contribute to slave labour and continuing social tensions."

The survivors of the latest shipwreck were mostly sub-Saharan Africans, but no further details were available, a Save the Children spokesman told Reuters. It was not clear exactly when the boat had capsized.

The number of boats carrying migrants aiming to reach the EU from Africa has picked up in recent weeks as fine spring weather makes the passage safer.

Save the Children, the IOM and other humanitarian organizations have called for the European Union to bolster its sea rescue operations before the migrant flows soar as they usually do in the summer months.

The EU border control agency, Frontex, estimates that there are 500,000 people in Libya waiting to make the crossing.

On Monday, 2,851 migrants were saved in rescue operations in the Mediterranean, the Italian coastguard said, adding to at least nine who died and 5,629 who were saved over the weekend.

Italy, which has the largest number of migrant arrivals in the EU, has become increasingly alarmed about the breakdown of law and order in Libya, which has greatly exacerbated the task of tackling the migrant flows. Libya is home to two rival governments, loosely aligned militia forces and a growing militant Islamist movement.

Separately, Frontex said on Tuesday that migrant traffickers had fired shots to prevent their wooden boat being confiscated after rescuers saved the 250 people it was carrying off the coast of Libya.

After the migrants had been transferred, a speedboat approached and its crew fired several shots into the air before the assailants sped away with the empty migrant boat, Frontex said.

Frontex said the episode marked the second time this year that armed smugglers had taken back a vessel used to transport migrants following a rescue in the central Mediterranean.

Additional reporting from Reuters.