Hundreds of migrants drowned when their boat capsized in the Mediterranean, survivors have told the BBC - although there is no official confirmation.

The 41 survivors say they were transferred to another vessel when it sank in the middle of the night.

They said that up to 500 people died, but coastguards in the region have been unable to confirm their accounts.

Numbers of migrants making the dangerous sea route from Libya to Italy have surged this year.

The survivors, from Ethiopia, Somalia, Sudan and Egypt, spoke to the BBC from the southern Greek city of Kalamata, where they are being held after their rescue.

According to the group, about 240 migrants left the Libyan port city of Tobruk heading for Italy.

Once out in the Mediterranean, they said they were transferred to a larger boat already packed with more than 300 people, which then capsized.

The survivors were then picked up by a cargo ship, whose crew told the BBC that the migrants initially refused to be handed over to the Greek coastguard as they were determined to get to Italy.