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International forces hammered the Libyan capital from the air for the third straight night Monday, according to reports on state television, though the nature of the targets was not immediately clear.

Anti-aircraft fire could be heard in Tripoli as the airstrikes were announced.

The strikes followed moves by rebels to advance into their former stronghold in the east to recapture lost ground, buoyed the air campaign launched over the weekend by Western allies, including Canada. Rebels also hailed news that an allied cruise missile smashed into Libyan leader Moammar Gadhafi's residential compound in Tripoli on Sunday.

The developments come as the U.S., Canadian and European alliance tries to determine if its three-day old air campaign should be helping the rebels' cause. If rebels continue to fight, Libya could end up a divided nation.

On Monday afternoon, U.S. President Barack Obama said that while his country's official policy is that "Gadhafi has to go," the UN mandate is more limited in scope, calling only for the protection of civilians.

"Our military action is in support of an international mandate from the Security Council that specifically focuses on the humanitarian threat posed by Col. Gadhafi to his people. Not only was he carrying out murders of civilians but he threatened more," Obama told reporters during a visit to Chile.

Also Monday, the United Nations Security Council denied Libya's call for an emergency meeting to halt what it called "military aggression" by France and the United States.

Last week, the Security Council passed a resolution approving a no-fly zone and authorizing the use of military force in order to protect civilians and halt Gadhafi's military offensive against the rebels.

That gave the go-ahead for an allied airstrikes on Libya, which began Saturday, mainly aimed at military air defences.

American officials are also maintaining it was not an American missile that destroyed a large part of one of the buildings in Gadhafi's compound on Sunday. It's still unclear whether the Libyan leader was in the building when it was bombarded.

In a statement Monday, Navy Vice Admiral William E. Gortney said Libya's air defences have been crippled and Gadhafi's ground forces have been stopped from approaching the rebel stronghold of Benghazi -- Libya's second largest city.

Gortney said air strikes have been "very effective," and the Pentagon believes Gadhafi's military is "under significant stress."

The Security Council resolution does not authorize attacks against Gadhafi himself.

Arab League supports actions

Also on Monday, United Nations Secretary-General Ban Ki-Moon emphasized that the decision to enforce a no-fly zone over Libya and to authorize military action against Gadhafi came at the recommendation of the Arab League.

He held a news conference with Arab League Secretary General Amr Moussa and said the goal was to stop Gadhafi's killing of civilians "which was totally unacceptable and strongly condemned all across the world."

Moussa emphasized the Arab League supports the UN's decision.

"We respect the Security Council decision and we have no objection to that because it has specified there will be no land troops to occupy Libya," Moussa said Monday, agreeing that the primary goal is to protect civilians.

On Sunday, there were reports that Moussa had complained that allied troops were shelling civilians, which didn't fit with the terms the Arab League had agreed to. However, an EU official later said Moussa was misquoted.

Rebels advance

While the rebels are grateful to have international help after weeks of fierce fighting against forces loyal to Gadhafi, it is still unclear if they are organized enough to make significant gains against the regime.

A group of several hundred rebel fighters advanced out of Benghazi Monday toward Ajdabiya, where pro-Gadhafi forces are hunkered down.

The rebels first made their to the nearby oil port town of Zwitina, which last week was the setting for heavy fighting between rebels and regime forces. Another skirmish broke out between the two sides Monday morning, and rebels were forced to retreat after suffering numerous casualties.

A rebel spokesperson called the advance "spontaneous," like many rebel movements. Khalid al-Sayah said army units that have crossed over to the rebel side will likely take over the organizing of rebel offensives.

"We don't want to advance without a plan," al-Sayah told the Associated Press. "If it were up to the army, the advance today would not have happened."

A group of about 150 rebel fighters eventually gathered outside Ajdabiya and were awaiting instructions to move on the city and fortify rebels who were cornered inside by pro-Gadhafi forces.

Fighting between the two sides also broke out Monday in Mistrata, Libya's third-largest city and one of the few rebel strongholds in the western part of the country. Pro-regime forces have been hammering the town in recent days, cutting of much of the food and water supply for residents.

Gadhafi promises ‘long war'

On Sunday the Libyan government escorted journalists from at least two major news outlets to the scene of Sunday's rocket attack to view the damaged building said to be a Gadhafi command centre. The building stands near the Bedouin tent where Gadhafi has been known to entertain foreign guests.

Half of the multi-storey building was destroyed, and scraps of metal from the missile lay scattered across the area.

Explosions and anti-aircraft gunfire erupted in the Libyan capital earlier in the night, signalling another round of coalition air assaults.

Despite the international pressure, Gadhafi pledged Sunday to continue fighting. In a phone call to Libyan state television, he said his weapon depots had been opened up to allow civilians to arm themselves against the outsiders.

"We promise you a long war," Gadhafi said.

During a press conference at the Pentagon on Sunday afternoon, Gortney said naval ships had launched 124 cruise missiles at military installations along the Libyan coast, followed by air assaults from French, British and American fighter jets and bombers against Gadhafi ground forces and air defences.

Military forces from Spain, Belgium, Denmark, Qatar and United Arab Emirates are joining the military operation under U.S. command, Gortney said, but a coalition of countries would take over command in the coming days.