U.S. President Barack Obama warned Libyan leader Moammar Gadhafi to stop attacks on civilians and allow humanitarian aid into the country or face consequences from the international community that could include military intervention.

"Let me be clear, these terms are not negotiable," Obama told a news conference in Washington on Friday.

However, Obama was also clear that the United States would not deploy ground troops in Libya or use force beyond protecting people.

U.S. Secretary of State Hillary Clinton will attend meetings in Paris on the weekend with her counterparts from Canada, Britain, France and the Arab states to discuss Libya and further steps, Obama said.

Al Jazeera television reported that pro-Gadhafi forces were advancing quickly towards the rebel stronghold of Benghazi on Friday evening, despite Obama's warning. 

Rebels said Misrata, Libya's third-largest city and the last held by rebels in the west, and Zintan, a western mountain town, came under assault well after the ceasefire announcement.

A Misrata doctor, who spoke on condition of anonymity because he feared reprisals, said Gadhafi's snipers were on rooftops and his forces were searching homes for rebels.

"The shelling is continuing, and they are using flashlights to perform surgery. We don't have anesthetic to put our patients down," said the doctor, who counted 25 deaths since the morning.

Rebels also said Misrata and Ajdabiya, an eastern city, were surrounded by government forces.

But Libyan deputy foreign minister Khaled Kaim told reporters his government has not violated the ceasefire and that the army outside Benghazi  had no plans to enter the rebels' de facto capital.

"As for the presence of the army in Libyan cities, we consider that important for the security of citizens," he said. "It does not violate the ceasefire."

Earlier on Friday, Libya's foreign minister declared a ceasefire in the government's fight against rebel forces after the UN Security Council authorized a no-fly zone over the country.

With U.S., British, French and Canadian forces poised to attack Libyan air force installations to ensure airborne weapons stay on the ground, Libyan Foreign Minister Musa Kusa said his government was obliged to accept the resolution.

Gadhafi's government "takes a great interest in protecting all civilians … respecting all human rights," Kusa said at a news conference in Tripoli. "It also recognizes its obligation to  protect all foreigners and their assets."

Ceasefire status unclear

However, Mustafa Gheriani, a spokesman for the national opposition council based in Benghazi, dismissed the ceasefire announcement, claiming Gadhafi's forces were still shelling the eastern city of Ajdabiya and Misrata, the last rebel-held city in the western half of the country.

To be sure, the Libyan foreign minister's conciliatory tone differed dramatically from statements hours earlier by Gadhafi.

Gadhafi told a Portuguese TV interviewer in Tripoli that the UN had no right to intervene in Libyan affairs.

"This is craziness, madness, arrogance," he said in a Reuters transcript. "If the world is crazy then we'll be crazy too .… We'll answer them, we'll respond to them, we'll make their lives hell as well, because this is injustice.''

Kusa said the government wanted to "take the country back to safety and security for all Libyans."

'All necessary measures' authorized

On Thursday, the UN Security Council authorized "all necessary measures" to stop attacks on civilians in Libya — including strikes by sea and air — hours after Gadhafi vowed to launch a final assault and crush the weeks-old rebellion against him.

Libya closed its airspace to all traffic Friday after the Security Council passed the no-fly resolution and authorized the use of force to protect Libyan citizens from their government.

Europe's air traffic control agency, Eurocontrol, told airlines "the latest information from Malta indicates that Tripoli [air control centre] does not accept traffic."

In Ottawa on Friday, Prime Minister Stephen Harper announced the country will deploy six CF-18 fighter jets to help enforce the no-fly zone, joining HMCS Charlottetown, which is en route to the Mediterranean Sea off Libya.

The jets were deployed Friday afternoon from a Quebec military base. They took off in snowy weather from CFB Bagotville, along with 150 personnel. Two C-17s from CFB Trenton are being used to transport the personnel. CBC News has learned the jets, pilots and support personnel will be based at an airbase in Trapani, on the west coast of Sicily in Italy.

Harper announced Friday morning his government authorized the deployment to support the UN resolution.

Reporting from Cairo, the CBC's Nahlah Ayed said the no-fly zone is an unusual situation.

"It’s not often that the UN allows military intervention, and certainly it is a sensitive subject here in the Middle East," she said. "On the other hand, though, the Arab League has given its OK for a no-fly zone."

Ayed said there appeared to be widespread sympathy for the UN action, especially with the Arab League involved.

"But for that sympathy to continue, for that sort of allowance of a foreign intervention, for the Arab world, it has to be short and focused, and it has to be successful as quickly as possible," she said.

'Track record of violence'

Harper as well as British Prime Minister David Cameron, French President Nicolas Sarkozy and Arab leaders will meet in Paris on Saturday to discuss immediate action against Libya.

Cameron said the U.K. will start moving fighter jets to bases from which they can help enforce the no-fly zone over Libya.

"Britain will deploy Tornadoes and Typhoons as well as air-to-air refuelling and surveillance aircraft," he told Parliament.

Speaking at a public meeting later, Cameron said three things were needed for the UN to act: "a need on the ground, strong regional support and a clear legal basis for anything that was proposed.

"These three conditions have now been met."

The prime minister also reminded his audience of Pan Am Flight 103, which exploded over Lockerbie, Scotland, on Dec, 21, 1988, killing 243 passengers and 16 crew. A Libyan was found to have been responsible, and many blamed Gadhafi.

"We must also remember that Gadhafi is a dictator who has a track record of violence and support for terrorism against our country and against Scotland specifically. The people of Lockerbie, just 100 miles [160 kilometres] away from here, know what he’s capable of."

With files from The Associated Press