Libyan rebels shot down a warplane that was bombing their eastern stronghold Saturday as the opposition accused Muammar Qaddafi's government of defying calls for an immediate cease-fire.

An Associated Press reporter saw the plane go down in flames outside Benghazi early Saturday, sending up a black cloud of smoke after the city came under attack. The sound of artillery and crackling gunfire was heard in the distance.

The leaders of Britain, France, Germany and the chiefs of the United Nations and Arab League are preparing to convene at Paris Saturday for an emergency summit regarding actions to enforce a Security Council resolution demanding that Col. Muammar al-Qaddafi obliges to a cease-fire against rebels.

U.S. Secretary of State Hillary Clinton also plans to travel to Paris for the meeting of allies after the U.S. ambassador to the U.N accused Qaddafi of violating the Security Council resolution.

France's ambassador to the United Nations, Gerard Araud, told BBC Newsnight that he expected military action to begin in Libya within hours of the Paris meeting.

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British Prime Minister David Cameron, one of the most enthusiastic backers of a no-fly zone, said Britain would send Typhoon and Tornado fighter jets to air bases "in the coming hours" so they would be in position to stop Qaddafi's forces from mounting air strikes against rebels in Benghazi. And NATO AWACS planes flying off the Libyan coast are already providing 24-hour surveillance of the situation in the air and on the battlefields.

Despite the warnings and a state declared cease-fire, pro-Qaddafi forces are advancing quickly towards Benghazi, Al-Jazeera reports.

Shelling has been heard on the outskirts of a rebel-held city of Benghazi in eastern Libya and a fighter jet was seen overhead.

Two columns of smoke were rising just outside the city on Saturday. Further details were not immediately available.

A correspondent for Al-Jazeera reported that Qaddafi's troops are also facing off against rebel forces in the towns of Al-Magroun and Slouq, about 30 miles from the city.

In an interview with CNN, Ambassador Susan Rice said Qaddafi faced "swift and sure consequences including military action."  It wasn't immediately clear if the U.N. was preparing to take action.

Earlier Friday, Obama made his first comments about the crisis in Libya since the U.N. Security Council voted late Thursday to authorize a no-fly zone and "all necessary measures" to protect the Libyan people from Qaddafi's bloody crackdown on rebels.

Obama said Qaddafi must implement a cease-fire, stop his troops from advancing on rebel strongholds and allow humanitarian assistance to reach Libya.

"Let me be clear, these terms are not negotiable," he said in a brief appearance at the White House. "If Qaddafi does not comply with the resolution, the international community will impose consequences and the resolution will be enforced through military action."

France, Britain, and Arab states have joined the U.S. and are threatening military actions if Qaddafi does not comply with demands to reconnect electricity and water in Libya communities.

"Humanitarian assistance must be allowed to reach the people of Libya. These terms are not negotiable. If Qaddafi does not comply with the resolution, the international community will impose consequences, and this resolution will be enforced through military action," the U.S., France, Britain, and Arab states said in a joint statement.

French President Nicolas Sarkozy also said in a statement that Qaddafi forces should pull out of Misrata, Zawiya, and Ajdabiya, Reuters reports.

A senior foreign ministry official said the government forces around Benghazi have no plans to attack the city and their presence does not violate the cease-fire, according to Reuters.

Obama said that while the U.S. will help impose the no-fly zone, he would not be sending U.S. ground troops.

"We are not going to use force to go beyond a well-defined goal -- specifically, the protection of civilians in Libya," he said.

Just hours after the U.N. vote, Libya declared an immediate cease-fire Friday and stopped all military operations.

But Secretary of State Hillary Clinton said that the Qaddafi regime would have to back up its declaration of a cease-fire with action.

"We are going to be not responsive or impressed by words, we have to see actions on the ground and that is not yet at all clear," Clinton said. "We will continue to work with our partners in the international community to press Qaddafi to leave and to support the legitimate aspirations of the Libyan people."

Clinton said the goal remains to get rid of Qaddafi.

"Col. Qaddafi's refusal to hear the repeated calls up until now to halt the violence against his own people has left us with no other choice but to pursue this course of action," she said.

The Obama administration has been facing mounting questions and criticism about its handling of the Libyan crisis, and Qaddafi's almost immediate declaration of a cease-fire following the U.N. authorization is almost certain to raise further questions about the White House's lack of leadership.

Obama met with congressional leaders from both parties on Libya before he made his statement. Sources with knowledge of the meeting said Obama told them that no U.S. troops and no U.S. attack aircraft will be directly involved in enforcement of the no-fly zone.

Lawmakers stood behind Obama's remarks.

"President Obama's stern ultimatum to Qaddafi is the right message," Sen. John Kerry, D-Mass., said. "The bottom line remains that Qaddafi has lost all legitimacy and determined international pressure will remain imperative to ensure that the will of the Libyan people prevails."

House Minority Whip Steny Hoyer, D-Md., said, "The president has indicated that the U.S. will assist international efforts to protect civilians and end the violence. I support this action and have urged the administration to take a supportive role in this effort."

One senior military official said Obama has essentially "drawn a line in the sand," warning Qaddafi not to step over it.

The U.S. backing for international action comes after several administration officials questioned the plan for providing aerial cover, with the Pentagon perhaps the most vociferous in its skepticism. It has described the no-fly zone as a step tantamount to war, and a number of U.S. officials have expressed fears that involvement in Libya could further strain America's already stretched military and entangle the country in an expensive and messy conflict in another Muslim country.

The U.S. has positioned a host of forces and ships in the region, including submarines and destroyers and amphibious assault and landing ships with some 400 Marines aboard. Britain announced Friday that it would send fighter jets and France was also making plans to deploy planes.

But the announced cease-fire has halted their momentum.

"Qaddafi is a smart operator...he has clearly read the U.N. Security Council resolution and now, with this cease-fire, the dynamic has changed again," a senior U.S. official told Fox News.

The official said there are constant talks with European allies about the next steps. While the U.S. and its allies are moving ahead with preparations, action is not imminent now because of Qaddafi's move.

It is highly unlikely Qaddafi would take some sort of "off-ramp" to leave his country a this point, the official said, but negotiation teams are being set up to deal with the dictator.

Fox News' Bret Baier, Carl Cameron, Reuters, Al-Jazeera and The Associated Press contributed to this report.