The United Nations authorized military strikes to curb Libyan leader Muammar Gadhafi, hours after he threatened to storm the rebel bastion of Benghazi overnight, showing "no mercy, no pity."

"We will come. House by house, room by room," Gadhafi said in a radio address to the eastern city late on Thursday.

Open gallery view The United Nations Security Council at the UN headquarters in New York on April 16, 2010. Credit: Reuters

Al Jazeera television showed thousands of people listening to the speech in a central Benghazi square, and then erupting in celebration after the UN vote, waving anti-Gadhafi tricolors and chanting defiance of the man who has ruled for four decades. Fireworks burst over the city and gunfire rang out.

The UN Security Council, in emergency session, passed a resolution endorsing a no-fly zone to halt government troops now around 100 km (60 miles) from Benghazi. It also authorized "all necessary measures" -- code for military action -- to protect civilians against Gadhafi's forces.

But time was clearly running short for the city that has been the heart of Libya's month-old revolution.

French diplomatic sources said military action could follow within hours, and could include France, Britain and possibly the United States and one or more Arab states; but a U.S. military official said no immediate U.S. action was expected.

While other countries or NATO may play roles in military action, U.S. officials expect the United States with its extensive air and sea forces would do the heavy lifting in a campaign that may include airstrikes on tanks and artillery.

Gadhafi warned Benghazi residents that only those who lay down their arms before his advancing troops would be spared the vengeance awaiting 'rats and dogs'.

"It's over. The issue has been decided," Gadhafi said. "We are coming tonight...We will find you in your closets."We will have no mercy and no pity."

Residents said the Libyan air force unleashed three air raids on the city of 670,000 on Thursday and there has been fierce fighting along the Mediterranean coastal highway.

Ten of the Council's 15 member states voted in favor of the resolution, with Russia, China and Germany among the five that abstained. There were no votes against the resolution, which was co-sponsored by France, Britain, Lebanon and the United States.

Apart from military action, it expands sanctions against Gadhafi and associates imposed last month. Among firms whose assets it orders frozen are the Libyan National Oil Corp and the central bank.

U.S. President Barack Obama called British and French counterparts David Cameron and Nicolas Sarkozy and agreed to coordinate closely on their next steps.

Libya said the resolution, which also demands a ceasefire by government forces, was not worth the paper it was written on.

Rebel National Council head Mustafa Abdel Jalil told Al Jazeera television air strikes, beyond the no-fly zone, were essential to stop Gadhafi.

"We stand on firm ground. We will not be intimidated by these lies and claims... We will not settle for anything but liberation from this regime."

It was unclear if Gadhafi's threat to seize the city in the night was anything more than bluster. But at the very least it increased the sense that a decisive moment had arrived in an uprising that only months ago had seemed inconceivable.

Some in the Arab world sense a Gadhafi victory could turn the tide in the region, weakening pro-democracy movements that have unseated autocrats in Tunisia and Egypt and raised mass protests in Bahrain, Yemen and elsewhere.

Gadhafi's Defense Ministry warned of swift retaliation, even beyond Libyan frontiers, to any military action against the oil-exporting nation.

"Any foreign military act against Libya will expose all air and maritime traffic in the Mediterranean Sea to danger and civilian and military (facilities) will become targets of Libya's counter-attack," the ministry said in a statement.