Hundreds of people are said to have died from the two boats carrying about 500 migrants which sank off Libya.
According to a Libyan official, about 200 people have so far been rescued.
The first of the boats which capsized on Thursday had 50 people on board, while the second was carrying over 400 people.
According to the United Nations (UN), about 2,400 migrants have died trying to cross the Mediterranean to Europe this year.
More than 100,000 others are said to have landed in Italy, while another 160,000 have crossed to Greece.
Meanwhile report suggests that more than 1,800 migrants have died in the Mediterranean in 2015. This is a 20-fold increase on the same period in 2014.
