A rebel attack against a military compound outside Syria’s capital on Wednesday offered the most tangible evidence yet that the country is sliding into armed conflict as regional powers issued an ultimatum for President Bashar al-Assad’s government to stop killing civilians.

Although the attack near Damascus does not appear to have been particularly effective, the target was highly symbolic: a compound of the Air Force Intelligence, which is renowned for its pursuit and torture of activists.

So, too, was the timing. After months of equivocation, Arab leaders are closing ranks on Assad, in part out of concern that the eight-month-old uprising against Assad’s rule is descending into an armed struggle that could spin beyond Syria’s borders.

But Assad’s loss of Arab support appears only to be accelerating the push to arms, by giving his opponents hope that they will soon receive international help. It also may be interpreted as a signal to members of Syria’s armed forces that now is the time to defect, said Fawaz Gerges, director of the Middle East Center at the London School of Economics.

“This is extremely dangerous,” he said. “We’re witnessing the emergence of a potent armed insurgency that could really plunge the region into conflict.”

The insurgency is coalescing around an entity called the Free Syrian Army, a group of defected army officers who fled to Turkey and proclaimed their existence in a YouTube video in July.

The group says it represents as many as 10,000 defected soldiers who are operating in small groups scattered around Syria. It asserted responsibility for Wednesday’s assault on the Air Force Intelligence building in a posting on its Facebook page, saying the strike was intended to “send a message to the regime that the Free Syrian Army can hit anywhere and anytime.”

Diplomats suspect that the number of defectors may be far smaller and that the group also comprises civilians who have taken up arms.

But Col. Malik al-Kurdi, the Free Syrian Army’s deputy commander, said in a telephone interview from Turkey that defections have risen in recent days in response to the Arab League’s decision Saturday to suspend Syria if it does not stop violence against protesters.

Kurdi said the rebel group is pushing Arab leaders to go further, toward the creation of a buffer zone along the Turkish border where a real rebel army can be formed and a no-fly zone imposed.

Neither Western nor regional powers have shown any inclination for military intervention in volatile Syria, but Kurdi said he is confident that it will eventually come.

“We are powerful and we can impose the reality of our power to push the Arab League,” he said.

In the suburbs of Damascus, where Wednesday’s attack occurred, protest organizers hailed the evidence that the Free Syrian Army is emerging as a force to challenge the regime.

“So many people here support the Free Syrian Army, but we need a protected area where it can organize,” said an activist who uses the name Dima, speaking via Skype. “When we started our revolution we were hoping we could remove this regime by peaceful means, but unfortunately we are now 100 percent sure we cannot do this.”

World leaders are still holding out for a peaceful conclusion to the revolt, and are hoping that the escalating diplomatic pressure and growing isolation will force Assad to change course. France withdrew its ambassador to Syria on Wednesday, becoming the first European Union nation to do so. France said it will again try to introduce a resolution condemning Syria at the U.N. Security Council. Russia and China have blocked past efforts to sanction Syria at the United Nations.

The Obama administration, which has called for Assad to step aside, said that U.S. Ambassador Robert Ford is expected to return to Damascus later this month. He was pulled back to the United States in October in response to what U.S. officials said were “credible threats” to his safety.

The three-day deadline to stop the violence or face sanction was issued at a gathering of Arab League foreign ministers in the Moroccan capital, Rabat. Although the ultimatum gives Assad a reprieve from the suspension that had been set to take effect, the ministers signaled that their patience is running out.

“We are close to the end of the road as far as the efforts on this front are concerned,” Hamad Bin Jasim al-Thani, Qatar’s foreign minister, told reporters.

Wednesday’s attack on the Air Force Intelligence compound was only one of several attacks in recent days. Kurdi said the attackers received help from sympathetic officers inside before beginning a four-pronged attack.

“We used [rocket-propelled grenades] and machine guns,” he said. “After hitting the building, we pulled out and we had no casualties. But there were many casualties inside the building because ambulances were coming in and out to take them.”

Residents of the area reported hearing explosions and gunfire around 2 a.m. One said that the only damage appeared to be some broken windows.

Another attack, purportedly on Monday in the southern province of Daraa, was brought to light by a video posted on YouTube in which civilians are seen milling around a blazing armored personnel carrier.

Whether armed rebellion will work where peaceful protests have failed is in question, however. The Syrian government has from the outset maintained that the uprising is the work of what it calls “armed gangs.” Now that some members of the opposition are fighting back against government assaults, the regime may feel justified in using even greater force.

The United States warned on Wednesday that violence “really plays into Assad’s and his regime’s hands,” State Department spokesman Mark Toner said.

“This was a peaceful movement from its inception, and it’s only because of the regime’s repeated and brutal campaign of violence against innocent protesters that we’ve seen the country move down this very dangerous path,” he added.

It is unlikely that the insurgents will be able to acquire enough weaponry to take on the government, which commands the loyalties of a sizable percentage of the population and a strong, well-equipped army, Gerges said.

“If it turns into an armed insurgency, it will be a prolonged conflict,” he said. “And unless a there’s a major shift in the balance of power, no one will be able to dislodge this regime. Maybe in a year, two, three or four years.”