Italy has mounted a rescue mission after a boat thought to be carrying as many as 120 illegal immigrants sank 10km off the coast of the island of Lampedusa.

An Italian navy ship spotted the vessel before it began to list, and was able to rescue around 70 people.

Coast guard vessels and helicopters later joined the operation, but the bodies of at least 10 people have so far been recovered. Forty are still missing.

It appears that a sudden shift of the people on board when the navy ship approached, toppled the 10-metre boat and everyone on board was flung into the sea. There were five women on board, and at least four of them are among the dead.

The Italian police has arrested five Libyans who are assumed to be among the people traffickers. The boat left the Libyan port of Al Zuwara three days ago.

Lampedusa, one of Italy’s southernmost islands, is a magnet for migrants.

Many Africans attempt the journey, often on overcrowded and unsafe boats.

Italian interior minister Giuliano Amato said: “This is not just a tragedy, this is a grave crime.”

The Italian interior ministry said 178 boats with a total of 10,414 people on board arrived in Lampedusa between 1 January and 31 July this year, compared with 6,901 illegal immigrants during the same period in 2005, according to news agency AFP.

Up to 17 illegal immigrants, believed to be from Africa, were reported missing off the coast of Malta after their boat sank on 26 July.