The United States, in a sharp shift in tone, wants the United Nations to authorize not just a no-fly zone to aid Libyan rebels but also air strikes against Libyan tanks and heavy artillery, U.S. officials said Thursday.

The move toward a tougher stance in favor of military action comes after an extended internal debate within the Obama administration over how to stop Libyan leader Moammar Gadhafi's battle to put down a rebellion that has threatened his 30-year reign.

The Libyan opposition has appealed for immediate assistance to prevent the rebel capital of Benghazi from falling to forces loyal to Gadhafi, and the question facing President Barack Obama and other world leaders was whether the action they planned to take would come in time.

U.S. officials said the United States has concluded a "no-fly" zone should be adopted and other measures that go well beyond a no-fly zone should be taken, including air strikes against Libyan tanks and heavy artillery.

Secretary of State Hillary Clinton said she hoped the Security Council would vote "no later than Thursday," and that a no-fly zone would require bombing targets inside the country.

"A no-fly zone requires certain actions taken to protect the planes and the pilots, including bombing targets like the Libyan defense systems," Clinton said as she neared the end of a Middle East trip dominated by worries about Libya, where a rebel offensive is apparently flagging.

The United States is also seeking U.N. authorization for other steps under consideration, including diverting frozen Gadhafi assets to Libyan rebels for buying weapons and tightening a Libyan arms embargo.

No 'boots on the ground'?

The United States supports international measures in Libya that are "short of boots on the ground," Undersecretary of State William Burns said on Thursday.

Burns also told the Senate Foreign Relations Committee that a no-fly zone over Libya could have "an important, positive, practical" effect, but it was still necessary to consider other measures.

He said Washington is concerned Gadhafi could "return to terrorism and violent extremism" and create turmoil in the Middle East.

Pentagon officials have made clear their wariness of instituting a no-fly zone with U.S. forces already engaged in Iraq and Afghanistan and a massive relief operation under way in Japan.

Any military plan adopted must have active participation by Arab League nations.

"They have to do more than just support it," a senior official said.

Gadhafi's forces making 'significant strides'

Libyan troops pushed forward toward the insurgent stronghold of Benghazi on Thursday and launched air raids on its outskirts as Washington raised the possibility of air strikes to stop Moammar Gadhafi's forces.

Gadhafi's forces, which have launched a major counter-attack against the rebels, have made "significant strides on the ground" and are about 100 miles from Benghazi, Undersecretary of State William Burns said.

The Libyan army said it would halt its operations on Sunday to give the rebels a chance to surrender, Al Arabiya television reported.

Libyan state television said government troops had taken Zueitina, an oil port on the coastal highway 80 miles from Benghazi, but the rebels said they had surrounded the pro-Gadhafi units on the approaches to the town.

In Benghazi, the city where the revolution began, residents and a rebel spokesman reported three air strikes on the outskirts, including at the airport, and another air raid further south.

Residential areas of Ajdabiyah, a strategic town on the coast road to Benghazi, saw heavy fighting on Thursday and around 30 people were killed, Al Arabiya reported.

On the approaches to Ajdabiyah, burned-out cars lay by the roadside while Libyan government forces showed the foreign media artillery, tanks and mobile rocket launchers -- much heavier weapons than those used by the rebels.

In Libya's third city, Misrata, about 130 miles east of Tripoli, rebels and residents said they were preparing for a new attack by Libyan troops, who had shelled the coastal city overnight. A government spokesman said Gadhafi's forces expected to be in control of Misrata by Friday morning.

Gadhafi: 'Armed thugs' occupying Benghazi

In an interview with the French daily Le Figaro, Libyan leader Moammar Gadhafi said his troops' aim was to liberate the people from "the armed gangs" that occupy Benghazi.

"If we used force, it would take just a day. But our aim is to progressively dismantle the armed groups, through various means, such as encircling cities or sending negotiators."

Asked if dialogue with the rebels was possible, he repeated his assertion that they were linked to the al-Qaida Islamic militant group.

"These are not people with whom we aim to talk, as al-Qaida does not talk with anybody."

On the fate of the rebel leadership, he said: "It is quite possible they will flee. Anyway, it's not really a structure. It has no value."

MSNBC contributed to this report.