Nato aircraft hit eight warships of Libyan leader Muammar Gaddafi's forces overnight on Friday in the ports of Tripoli, Al Khums and Sirte, the Atlantic alliance said in a statement.

"Nato and coalition air assets continued their precision airstrikes against pro-Gaddafi regime forces overnight with a coordinated strike against pro-Gaddafi forces in the ports of Tripoli, Al Khums and Sirte," the statement said.

"Overnight, Nato aircraft hit pro-Gaddafi warships, striking eight vessels," it added.

"All Nato's targets are military in nature and are directly linked to the Gaddafi regime's systematic attacks on the Libyan people," said rear-Admiral Russell Harding, deputy commander operation unified protector.

"Given the escalating use of naval assets, Nato had no choice but to take decisive action to protect the civilian population of Libya and Nato forces at sea," he further added.

Fresh explosions were heard in the Libyan capital Tripoli early Friday hours after Nato air strikes targeted the city's port, with a ship still ablaze after the raid.

An AFP journalist, part of a group sent by the authorities to a bridge facing the port about a kilometre (less than a mile) away, was unable to determine whether the blazing ship was a military or civilian vessel.

The Nato statement said: "Over the past couple of weeks we have witnessed indiscriminate mining and the escalating use of force by pro-Gaddafi maritime forces.

"This has directly disrupted the safe flow of desperately needed humanitarian assistance and put Nato forces at risk. This development of pro-Gaddafi tactics has also demonstrated a clear intent to attack Nato forces.

"Last night, Nato took deliberate action in carefully planned and coordinated responses to demonstrate our resolve to protect the civilian population of Libya, using appropriate and proportionate force."

Rear-Admiral Harding said: "All the vessels targeted last night were naval warships with no civilian utility."

Earlier, regime spokesman Mussa Ibrahim told a press conference in the Libyan capital: "I have just learned that the port of Tripoli is now being targeted by Nato air raids. I am told that a boat has been hit."

He did not give further details on the vessel, but told the journalists: "Whatever the ship that has been hit, it is clearly a message sent by Nato to the international maritime companies not to send any more vessels to Libya."

Witnesses told AFP that they heard at least four explosions in the port and saw columns of smoke rising from the area.

"Military and civilian sites are currently the targets of raids by the colonialist Crusader aggressor," Libyan state television said.

The Nato statement said that on April 29, "pro-Gaddafi forces used pro-Gaddafi maritime assets to mine the entrance to the port of Misrata. On a further three occasions, pro-Gaddafi maritime assets have been intercepted by Nato ships. This included an incident where Nato interdicted a booby-trapped vessel leading to the discovery and subsequent destruction of one tonne of explosives at sea.

"Since the start of Nato's mission, we have been vocal and pro-active in instructing pro-Gaddafi forces to lay down their arms. We have communicated our desire for them to move away from military equipment, military installations and maritime assets," the statement said.

"Nato has constantly adapted to the rapidly changing and dynamic situation in Libya and at sea. This is a complex campaign, which is being conducted within the United Nations Security Council Resolutions 1970 and 1973. This mandate legally authorised the use of all necessary measures to protect the civilian population of Libya."

Tripoli is targeted nearly daily with air raids by the international coalition, which launched strikes on March 19 to prevent strongman Gaddafi's forces from attacking civilians.

Nato took over command of the operation on March 31.