Italian coast guards today rescued more than 300 badly dehydrated refugees fleeing Libya on an overcrowded 20-metre boat that had been drifting for more than 36 hours, officials said.

Three of the refugees including a pregnant woman were flown off the boat by helicopter directly to a hospital on the island of Lampedusa, where tens of thousands of African migrant workers from Libya have landed in recent weeks.

Italian news agency ANSA cited the three refugees as saying that "dozens and dozens of people" had died of thirst and hunger and were thrown overboard.

A coast guard spokesman, Vittorio Alessandro, could not confirm the report.

Alessandro said the refugees were rescued some 90 nautical miles south of Lampedusa. He said the boat was expected to arrive in Lampedusa later today with coast guards escorting it.

A Cypriot boat initially spotted the refugees and alerted Italian coast guards after some of the people on board tried to swim across to them in high seas.

The crew threw life rafts into the water to stop the refugees from drowning.

An Italian helicopter then lowered supplies to the refugees on a basket but was forced to abandon the operation after some refugees tried to cling to it.

Twenty-five African refugees from Libya apparently choked to death in the engine room of a boat that arrived on Lampedusa on Monday. Some 250 refugees drowned in April as Italian coast guards attempted to rescue them.