At least 17 people died when a boat carrying hundreds of migrants sank in waters between Libya and southern Italy, navy officials say.
Some 200 others were rescued from the boat, which went down south of the Italian island of Lampedusa.
This comes a day after it emerged that 36 migrants drowned last week when their boat sank off the Libyan coast.
Libya is used as a departure point by many African migrants trying to enter the European Union illegally.
The boat sank on Monday about 185 km (115 miles) south of Lampedusa, Italy's Ansa news agency said.
An Italian navy spokesperson said it was not clear how many people were on board, so the number of missing passengers was unknown, AP news agency reported.
A tug boat servicing nearby oilrigs saw that the migrant boat was in trouble. As it went to help, the vessel capsized.
The Italian navy has sent a frigate and a helicopter to help in the search for survivors.
Separately, Libya's navy said it had rescued 340 migrants from another boat after it began to take in water off the coast of the western town Sabratha, AFP reported.
On Sunday, Libyan officials said at least 36 migrants from various sub-Saharan African countries drowned when their boat sank off the Libyan coast last week.
Late last year, hundreds of migrants drowned when two boats sank in waters off Lampedusa. Italy has since stepped up navy and coastguard operations.
Unseaworthy, overcrowded vessels are continually setting out from Libya, carrying people who hope for better lives in Europe after fleeing war and poverty in various parts of Africa and the Middle East, the BBC's Alan Johnston in Rome reports.
The Italian government has accused the European Union of failing to do enough to address problems created by the flow of migrants, our correspondent adds.
Italy's Foreign Minister Federica Mogherini said on Monday: "We must collaborate with our European partners to manage the flow of people crossing the Mediterranean.
"Most of all we must find long-term policies to address the causes of flows, especially the prevention and management of crisis around the Mediterranean," she added.
Interior Minister Angelino Alfano said: "The Mediterranean is not an Italian border but a European border".