About 400 migrants died in an attempt to reach Italy from Libya when their boat capsized, survivors said on Tuesday, the latest tragedy in the Mediterranean where the death toll from shipwrecks has surged this year.

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 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.