Loud explosions and anti-aircraft fire have rocked the Libyan capital for the third night as an international coalition enforces a no-fly zone over the country and targets forces loyal to leader Moammar Gadhafi.

Al-Jazeera correspondent Anita McNaught, reporting from Tripoli, said the explosions were heard about 10 minutes apart soon after 9 p.m. and that two naval bases had apparently been hit.

"We could see an area of the port on fire, substantially on fire, two big blazes," said McNaught.

Al-Jazeera reported that Mussa Ibrahim, a Libyan government spokesman, told a news conference that coalition attacks had killed civilians in port areas and at Sirte airport, and had also targeted the southern town of Sebha, a stronghold of Gadhafi's Guededfa tribe. The claim could not be independently confirmed.

Al-Jazeera television reported earlier that international coalition forces had struck radar installations at two air bases belonging to Gadhafi forces in eastern Libya, east of rebel-held territory in Benghazi.

Earlier on Monday, U.S. President Barack Obama said the coalition air campaign over Libya is not aimed at removing Gadhafi from power, despite American policy assertions that the Libyan ruler "has to go."

Speaking to reporters during a visit to Chile, Obama insisted the purpose of the military mission is in response to the humanitarian threat Gadhafi poses to his people.

"There are a whole range of policies that we are putting in place that have created one of the most powerful international consensuses around the isolation of Mr. Gadhafi and we will continue to pursue those," Obama said.

"But when it comes to the military action, we are doing so in support of UN Resolution 1973 that specifically talks about humanitarian efforts, and we are going to make sure we stick to that mandate."

Obama's comments echoed those of the U.S. commander of the coalition military mission, who said aircraft enforcing a no-fly zone over Libya are there to protect civilians and not to provide air support for rebel forces.

During a briefing at the Pentagon on Monday, Gen. Carter Ham said the coalition fired 12 more cruise missiles Monday at Libyan missile, command and air defence sites. Ham added there is no direct co-ordination with the anti-Gadhafi forces.

Airstrikes averted 'massacre': U.K. PM

Libyan soldiers survey the damage to an administrative building hit by a missile late Sunday in the heart of Moammar Gadhafi's Bab Al Azizia compound in Tripoli early Monday. (Jerome Delay/Associated Press)

The building in the Gadhafi compound was apparently targeted because it was believed to contain air defences, CBC's Nahlah Ayed said from Cairo on Monday.

Earlier in the day, British Prime Minister David Cameron stressed in Britain's House of Commons that through airstrikes, coalition forces helped avert what could have been "a bloody massacre in Benghazi."

Meanwhile, Defence Minister Peter MacKay said more CF-18s are on standby to deploy to Libya following Canada's first air mission on Monday, but no formal request for the additional aircraft has been made.

Four CF-18s and two CC-150 air-to-air refuellers flew as escorts for another country's aircraft during a bombing mission, but the Canadian aircraft did not fire their weapons, MacKay said during a briefing.

Rebels aim to gain ground

Meanwhile, anti-government rebels tried to reclaim key Libyan cities Monday after international forces launched fresh airstrikes overnight.

Rebels said they were trying to reclaim the city of Ajdabiya, which had been surrounded by government troops before the international campaign began.

"There are pro-Gadhafi forces inside fighting rebel fighters. There is fighting and shelling going on," said Ahmad Mohammed, 26, who returned from the front Monday.

Ayed said rebel fighters from Benghazi were also making their way to Ajdabiya.

New fighting also broke out Monday in Misrata, the last rebel-held city in western Libya, according to reports from Al-Jazeera and Al-Arabiya.

With files from The Associated Press