Hundreds of migrants locked in hold

Migrants wait to leave the island of Lampedusa, southern Italy. (AP)

A smuggler's boat crammed with hundreds of people overturned off Libya's coast as rescuers approached, causing what is feared to be the Mediterranean's deadliest known migrant tragedy.

Italian prosecutors said a Bangladeshi survivor told them 950 people were aboard, including hundreds who had been locked in the hold by smugglers.