Tobruk, Libya ? The U.N. Security Council on Thursday authorized “all necessary measures” to stop Moammar Gadhafi in Libya — including strikes by sea and air — hours after he vowed in harrowing terms to launch a final assault and crush the weeks-old rebellion against him.

The resolution, approved with the backing of the United States, France and Britain, imposed a no-fly zone over Libya and authorized force short of a ground offensive to protect its people from Gadhafi’s forces.

The U.N. action bans all flights in Libyan airspace in order to protect civilians. While it was unclear how the West might proceed, U.S. Secretary of State Hillary Rodham Clinton said earlier in the day that a no-fly zone would require bombing targets inside Libya, including some of its defense systems.

It was also unclear when any Western action would come. A British lawmaker said British forces could be mobilized as early as Thursday night. U.S. officials, speaking after a closed-door briefing in Congress, said they expected an attempt to ground Gadhafi’s air force could begin by Sunday or Monday and would probably involve jet fighters, bombers and surveillance aircraft.

After deliberating for weeks over what to do about Gadhafi, the West acted with sudden speed as it became clear Gadhafi would attempt to finally put an end to the rebellion.

Gadhafi, calling in to Libyan television on Thursday, said his forces would “rescue” the people of Benghazi, the Mediterranean port city that has become the capital and staging ground for the opposition. For those who resist, Gadhafi said, there would be “no mercy or compassion.”

“This is your happy day, we will destroy your enemies,” he said, warning the people of Benghazi not to stand alongside the opposition. “Prepare for this moment to get rid of the traitors. Tomorrow we will show the world, to see if the city is one of traitors or heroes.”

“Don’t betray me, my beloved Benghazi,” he said.

His ground forces were still about 80 miles south of the city on Thursday evening Libya time, so it was unclear whether they would move on the city as quickly as he suggested.

Speaking moments before in an interview with Portuguese television broadcast just before the vote at the U.N., Gadhafi pledged to respond harshly to U.N.-sponsored attacks. “If the world is crazy,” he said, “we will be crazy, too.”

At the U.N. headquarters in New York, the vote was 10-0. The United States, France and Britain had all pushed for speedy approval. “We had said all along that Gadhafi must go,” said British Foreign Secretary William Hague. “It is necessary to take these measures to avoid greater bloodshed.”

Five nations abstained, including Russia and China, which hold veto power on the council.

U.S. preparations

In Washington, officials said the Obama administration was readying plans to enforce the no-fly zone. The French prime minister said before the measure was passed that his nation would support military action within hours. Several Arab nations were expected to provide backup.

“Today the Security Council has responded to the Libyan people’s cry for help,” said Susan Rice, the U.S. ambassador to the U.N. “Colonel Gadhafi and those who still stand by him continue to grossly and systematically abuse the most fundamental of the human rights of his people.”

The United States already has warships positioned near Libya. After eight hours of closed-door talks on Wednesday, Rice said a no-fly zone now was not enough, saying it has “inherent limitations in terms of protection of civilians at immediate risk.”

In Britain, a lawmaker with knowledge of defense matters confirmed that British forces were on standby for air strikes and could be mobilized as soon as Thursday night. The lawmaker declined to be named because the Defense Ministry has not issued official confirmation.

French President Nicolas Sarkozy’s office issued a one-sentence statement at about 2 a.m. today Paris time saying he and Obama had spoken by phone about the resolution. Obama also spoke with British Prime Minister David Cameron.

Western countries have significant military assets nearby, including carriers in the Mediterranean, a large U.S. air base in Italy and a large British air presence on the island of Cyprus. In addition, allied Arab countries such as Jordan and Oman have planes and pilots often trained by the U.S., and American officials have made clear they want active involvement by Arab countries if any action is taken.