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 taken to a southern Italian port this morning, Save the Children reported.

Before this incident there had already been more than 500 deaths of migrants crossing the Mediterranean from Africa this year, up sharply from 47 in the same period of 2014, said the Geneva-based International Organisation for Migration (IOM).

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. In February, more than 300 drowned when attempting the crossing in cold weather and rough seas.

Save the Children, the IOM and other humanitarian organisations 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.

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 

Separately, the EU border control agency Frontex said today 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.