French fighter jets streaking over Libya on Saturday bombed a military vehicle and walled off a 600-square-mile sanctuary over the eastern city of Benghazi in the first military engagements in support of the no-fly zone authorized two days ago by the U.N. Security Council.

A French military spokesman confirmed the attack on units loyal to Libyan leader Moammar Gaddafi and said about 20 French planes were patrolling the skies over rebel-held Benghazi to halt an apparent effort by the Libyan army to quickly crush the opposition before foreign forces could intervene.

“A first target was engaged and destroyed,” the spokesman said. Local broadcast reports that four Libyan tanks had also been destroyed by French aircraft could not be independently confirmed.

The first shots fired in the U.N.-backed campaign came shortly after French President Nicolas Sarkozy confirmed in a televised address that French warplanes had entered Libyan airspace to prevent Gaddafi’s “murderous madness” against Libyan civilians.

“Today we are intervening in Libya with a U.N. Security Council mandate,” Sarkozy said.

The rapid deployment of warplanes came as Gaddafi appeared to defy U.N. demands for a cease-fire by escalating attacks against rebels dug in around Benghazi, a city of about 1 million people and the largest remaining opposition stronghold.

The French warplanes are expected to be joined in coming days by aircraft and logistical support from Britain, the United Arab Emirates, the United States and other countries under a U.N. mandate authorizing a broad use of force to prevent the slaughter of Libyan citizens.

Sarkozy spoke at the close of an emergency meeting in Paris that was described as an effort to project international unity and resolve against Gaddafi. U.S. officials had said that the meeting in Paris, which drew leaders from 22 countries, among them Secretary of State Hillary Rodham Clinton, would focus on developments in Libya and the next steps to take.

An administration official, speaking shortly before the news broke about the French intervention, said all the leaders recognized the urgency of the situation in eastern Libya.

“We’ve made clear what our expectations are, and we have also made clear that the international community is prepared to act if he doesn’t meet those expectations,” the official said.

Forces loyal to Gaddafi entered the eastern rebel stronghold of Benghazi early in the day after shelling and fierce fighting, a fresh act of defiance of U.N. calls for a cease-fire. Government troops in tanks and trucks entered Benghazi from the southwest, in the university area, and began to shell the city, including civilian areas. Intense fighting broke out in some enclaves. The city quickly became a ghost town, with residents fleeing or seeking cover in barricaded neighborhoods.

A warplane crashed down over Benghazi, and rebel leaders later claimed it as one of theirs. Although they said mechanical problems caused the crash, calls from mosques across the city suggested that friendly fire brought down the plane. “Don’t attack the airplanes, because these are our planes,” a mosque preacher urged over loudspeakers.

President Obama, speaking at an appearance in Brasilia with Brazilian President Dilma Rousseff, reiterated his call for the Libyan government to stop attacking rebels or face military action.

“Our resolve is clear: The people of Libya must be protected,” Obama said in brief remarks after a meeting with Rousseff. He did not take questions.

On Friday, Obama had warned that the Libyan leader faced imminent military action unless his troops were withdrawn from all disputed cities in the country. But the besieged town of Misurata, 130 miles east of Tripoli, was still coming under heavy artillery fire, residents said, and there were also reports of continued fighting around Ajdabiya, even farther to the east. The assaults on rebel-held towns took place despite government promises of a cease-fire.

In what appeared to be a desperate attempt to avert military action, Gaddafi sent two letters to international leaders, according to deputy foreign minister Khaled Kaim, who read the letters to journalists. One was a warm, conciliatory note to Obama, and the other was a sharply worded, menacing message to the United Nations, France and Britain.

To Obama, he wrote: “If Libya and the US enter into a war you will always remain my son, and I have love for you.” Libya is battling al-Qaeda, he said, seeking Obama’s advice. “How would you behave so that I can follow your example?” he asked.

In the other letter, addressed to U.N. Secretary General Ban Ki-moon and the leaders of France and Britain, he warned that the region would be destabilized if they pursued strikes against Libya. “You will regret it if you take a step to intervene in our internal affairs,” he wrote.

Libyan government spokesman Ibrahim Musa said rebels, not government forces, were the ones breaking the cease-fire by attacking military forces, the Associated Press reported. “Our armed forces continue to retreat and hide, but the rebels keep shelling us and provoking us,” he said.

The conditions set by Obama were more specific than those contained in a resolution approved a day earlier by the U.N. Security Council, suggesting that the United States and its allies are in no mood to countenance delays by a Libyan regime whose forces have recaptured large swaths of territory from rebels in recent days.

U.S. ships in the Mediterranean were preparing to bombard Libya’s air defenses and runways to clear the way for European and Arab forces to establish a no-fly zone throughout the country, according to U.S. and European officials. Fighter aircraft from France, Britain and the United Arab Emirates converged on bases in and around Italy to begin operations over Libya under the command and control of the United States at its naval base in Naples.

Also Saturday, Defense Secretary Robert M. Gates postponed by one day a scheduled trip to Russia so he could monitor developments in Libya, Pentagon press secretary Geoff Morrell said.

Friday was a day of bloodshed across the Arab world, as governments appeared increasingly willing to use arms to suppress the dissent that has mushroomed since the success of popular uprisings in Tunisia and Egypt earlier in the year. In Yemen, 47 people died when security forces opened fire on protesters in the capital, Sanaa, and at least five people were reportedly killed during a government crackdown on unrest in Syria, a strategically vital country that has been ruled with an iron fist by the Assad family for the past four decades.

In an address at the White House, Obama spelled out conditions that Gaddafi would have to fulfill if his country is to avoid military intervention under the provisions of a U.N. Security Council resolution adopted Thursday that authorizes the use of force to stop the violence.

“These terms are not negotiable,” Obama said. “If Gaddafi does not comply with the resolution, the international community will impose consequences and the resolution will be enforced through military action.”

Obama said that in addition to halting their advance on Ben­ghazi, the last remaining rebel stronghold in the east of the country, Libyan troops would have to pull back from the towns of Ajdabiya, Misurata and Zawiyah. He also demanded that Libya establish water, electricity and gas supplies to all areas.

A doctor at the hospital in Misurata said 16 civilians and 25 rebels had been killed in a government assault Friday, at least a dozen of them after the cease-fire supposedly went into effect. In Benghazi, a rebel spokesman said two people had been killed in Ajdabiya in attacks after the cease-fire was announced.

“Until this moment, they have not stopped attacking us. There will be no negotiations,” Khaled al-Sayeh told reporters Friday night. “The attacks are still happening at this moment.”

He said attacks were also taking place in the towns of Zintan and Zuwaytinah well after the cease-fire was announced.

In the area around Zuwaytinah, more than 90 miles south of Benghazi, jets streaked across the sky firing at targets, at least one helicopter flew low across the desert, and artillery bombardment could be heard for several hours Friday afternoon.

A tentative deadline for Gaddafi’s full compliance was set at midday Saturday after the Paris meeting.

It appeared clear that the Libyan government had been caught off-guard by the speed with which the Security Council moved to authorize the use of force, after weeks of indecision during which pro-Gaddafi forces made significant advances.

Kaim, Libya’s deputy foreign minister, denied that there had been any violations of the cease-fire since it was announced early Friday afternoon. He called on observers from four countries — Malta, China, Germany and Turkey — to send a fact-finding team to Libya to verify that the cease-fire is being observed.

U.S. officials expressed skepticism.

“We are not going to be responsive and impressed by words,” Clinton told reporters in Washington. “We would have to see action on the ground, and that is not yet at all clear.”

She added that “the final result of any negotiations would have to be the decision by Colonel Gaddafi to leave. But let’s take this one step at a time.”

Britain’s Cameron also said he was unconvinced by the cease-fire announcement, telling members of Parliament that Britain was already preparing to deploy fighter jets and other air support to the region.

“Our forces will join an international operation to enforce the resolution if Gaddafi fails to comply with its demand that he ends attacks on civilians,” he said.

A U.S. official with access to classified intelligence on Libya said the CIA and other American spy agencies monitoring Libya had seen evidence of continued fighting, from satellites and from sources inside the country.

“There are reports out of certain areas that fighting continues,” said the official, citing Misurata. The cease-fire “should be considered tenuous at best right now,” the U.S. official said, speaking on the condition of anonymity to discuss sensitive intelligence information.

Raghavan reported from Sultan, Libya, and Warrick from Washington. Correspondent Anthony Faiola in London and staff writers Karen DeYoung, Greg Miller and Craig Whitlock in Washington, Mary Beth Sheridan in Paris and Perry Bacon Jr. in Brasilia contributed to this report.