Italy has urged NATO to help rescue refugees fleeing Libya by sea and demanded a formal inquiry into the alliance's reported failure to assist a stricken boat on which dozens may have died of dehydration.

NATO's mandate in Libya should be adapted "to take into consideration the care and rescue of those who are forced to flee by boat, putting their lives at risk because of combat operations," the foreign ministry said in a statement.

Italy asked for assistance from a NATO vessel taking part in the naval embargo on Libya to rescue a boat with hundreds of refugees drifting off Libya and was refused, according to a foreign ministry source cited by ANSA news agency.

The refugees were eventually rescued by the Italian coastguard and taken to the island of Lampedusa, where tens of thousands of mainly African migrant workers fleeing Libya have arrived on rickety fishing boats in recent weeks.

Survivors said around 100 people had died during the journey.

The foreign ministry said it wanted an inquiry into "the controversy over the presumed failure to assist a boatload of migrants fleeing Libya."

ANSA reported that the NATO vessel asked to help was 28 nautical miles from the 20-metre boat carrying more than 300 refugees.

A NATO spokesman for naval operations, David Taylor, denied that NATO had failed to assist saying: "NATO always responds and intervenes in emergency situations in accordance with the requirements of international law."