About 400 migrants have died in an attempt to reach Italy from Libya when their boat capsized, survivors said. It's the latest tragedy in the Mediterranean where the death toll from shipwrecks has surged in 2015.

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, the Save the Children charity 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.

Traffickers fire shots at rescuers

On Monday 13 April 2015, 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 handled 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, the EU border control agency Frontex said 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.