Italian and Maltese ships – backed by helicopters – are assisting at the scene.
At least 120 people are said to have been saved, but bodies have also been reportedly spotted in the water.
Last week more than 300 people drowned when a boat carrying African migrants sank off Lampedusa.
Italy’s coast guard and Maltese officials said the boat capsized about 70 miles off Lampedusa on Friday.
The vessel was reportedly first spotted by the Maltese air force who then requested Italy’s help.
The Maltese authorities are co-ordinating the operation in the country’s territorial waters.
Italy’s Ansa news agency reports that bodies were seen in the water – but this has not been officially confirmed.
The nationalities of those on board the boat were not immediately known.
Lampedusa, which lies 180 miles off the coast of Africa, is a key destination for migrant vessels bound for Europe.
Last Thursday, more than 300 people – mostly Somalis and Eritreans – died when their boat sank near the island.
Only 155 people were rescued of more than 500 migrants.
Many of the island’s residents have long complained that EU and Italian authorities are not doing enough to deal with the thousands who come ashore.
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