The Italian coastguard says it's involved in a major rescue operation after a boat carrying some 700 migrants capsized in the Mediterranean Sea.

So far 28 people have been rescued.

Italian ships, the Maltese Navy and commercial vessels are all assisting with the rescue operation, which is in Libyan waters about 200km south of Italy's island of Lampedusa.

Maltese Prime Minister Joseph Muscat has tweeted that many people are feared dead.