NATO said on Friday its warplanes hit eight vessels of Muammar Gaddafi's navy, after US President Barack Obama predicted the Libyan strongman would 'inevitably' leave or be forced from power.

The authorities in Tripoli slammed Obama's comments as 'delusional', while strongly denying reports that Gaddafi's wife and daughter had fled to Tunisia and that Oil Minister Shukri Ghanem had defected.

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

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

"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," said Rear Admiral Russell Harding, deputy head of the NATO-led air war.

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

Harding insisted that all of the targets hit were military but government spokesman Mussa Ibrahim accused NATO of seeking to scare international shipping firms into steering clear of government-held ports.

"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," Ibrahim told reporters in Tripoli.

British aircraft targeted Al-Khums, the nearest naval base under the control of Gaddafi forces to the rebel-held city of Misrata, British defence staff spokesman Major General John Lorimer said.

"As well as hitting two corvettes in the harbour, the Royal Air Force Tornados successfully targeted a facility in the dockyard constructing fast inflatable boats, which Libyan forces have used several times in their efforts to mine Misrata and attack vessels in the area," Lorimer said.

"The destruction last night of the facility and a significant stockpile of the boats will reduce the regime's ability to sustain such tactics," he added.

NATO has increased the pressure on Gaddafi by hitting several command and control centres in Tripoli in recent days.

"This has limited Gaddafi's ability to give orders to his forces. It has also constrained his freedom of movement; effectively he's gone into hiding," NATO's Wing Commander Mike Bracken said in Brussels.

In Tripoli, the government spokesman described as 'delusional' Obama's prediction in a keynote speech on US Middle East policy on Thursday of the veteran Libyan leader's inevitable departure.

"Obama is still delusional -- he believes the lies that his own government and own media spread around the world," Ibrahim said.

"It's not Obama who decides whether Gaddafi leaves Libya or not. It's the Libyan people who decide their future."

In a major speech responding to the protest movements sweeping the Arab world, Obama said: "Time is working against Gaddafi.

"He does not have control over his country. The opposition has organised a legitimate and credible interim council," Obama said of the rebels' National Transitional Council based in their eastern stronghold of Benghazi.

"And when Gaddafi inevitably leaves or is forced from power, decades of provocation will come to an end, and the transition to a democratic Libya can proceed."

Obama's remarks were welcomed by the vice chairman of the rebel administration, Abdul Hafiz Ghoga.

"We welcome the statements made regarding the legitimacy of the body of the National Transitional Council," Ghoga said in Benghazi.

"We look forward to further support from the United States and the international community to help us further develop our democratic aspirations and for the development of our people."

Meanwhile, the family of South African photographer Anton Hammerl, missing for six weeks, revealed he had been shot dead by Gaddafi's forces on the front line between rebel-held eastern Libya and the mainly government-held west.

"Anton was shot by Gaddafi's forces in an extremely remote location in the Libyan desert," said a statement posted on Facebook.

"According to eyewitnesses, his injuries were such that he could not have survived without medical attention."

South African authorities only learned of Hammerl's death after the release on Wednesday of four other journalists who were with him when he was shot, South African Foreign Minister Maite Nkoana-Mashabane said.

"The journalists knew that he had been killed but decided for their own survival not to say anything in consular contacts and telephone conversations with their families."

In other developments, African Union leaders will gather for an extraordinary summit in Addis Ababa next week to discuss the Libyan conflict, the organisation announced.

"The session will be devoted to the consideration of peace and security in Africa in light of the challenges and crises facing the continent," a statement said.

Last month, pan-African body proposed a truce but this was rejected by rebels, who insisted on Gaddafi's departure.