A tenuous truce went into effect in Syria on Friday after the government and the rebel Free Syrian Army said they will observe a four-day ceasefire to coincide with a major Muslim holiday, amid reports of significant rebel gains in the strategically vital northern city of Aleppo.

But reports of scattered gunfire and clashes near a northern town cast into doubt the likelihood that the truce would last even beyond the first day.

In one incident, three anti government protesters were injured after security forces opened fire to prevent citizens from demonstrating as they emerged from a mosque, according to the Britain-based Syrian Observatory for Human Rights. Encouraged by the government’s promise to cease fire, protesters surged onto the streets in numerous towns and cities to stage anti-government demonstrations, in a reminder of the early days of the initially peaceful 19-month old uprising against President Bashar al-Assad’s rule.

The Observatory also reported that fierce clashes had erupted between government forces and rebels east of the strategic town of Maarat Numan in northern Idlib. Fighters with the extremist Jabhat al-Nusra Front, which has vowed not to observe the truce, attacked a checkpoint, prompting the army to shell a nearby village, the Observatory said.

A statement from the Syrian army flashed on state television Thursday evening said the cease-fire would last through Monday, in accordance with a proposal by U.N. special envoy Lakhdar Brahimi that both sides stop fighting for the duration of the Eid al-Adha holiday.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

Minutes later, the commander of the Free Syrian Army’s military council, Mustafa al-Sheik, told the television network al-Jazeera that if government forces stopped shelling towns and cities under opposition control, the rebels would abide by the cease-fire “out of respect for Brahimi.”

If implemented, the truce would mark the first pause since April in the intensifying violence in Syria, where more than 150 people on average are being killed daily, according to human rights and activist groups. It would also offer a glimmer of hope that the United Nations’ hitherto fruitless efforts to broker a broader diplomatic solution are not yet dead.

U.N. Secretary General Ban Ki-moon hopes that the cease-fire will build momentum toward political negotiations, his spokesman in New York said. In Washington, the State Department urged both sides to respect the truce.

But the Free Syrian Army leadership only loosely commands the hundreds of fragmented rebel groups battling Assad’s regime nationwide, leaving it unclear whether all of them would respond to the truce call. The radical Jabhat al-Nusra, which has been playing an increasingly significant role on the battlefield, has said that it will not comply.

“For now, no one can speak for the armed opposition,” Sheik said. “It only listens to the Syrian will.”

Momentum for rebels

The cease-fire announcement coincided with indications that the rebels had seized control of several areas in the heart of the front-line city of Aleppo, where government forces have been battling since July to hold back a rebel offensive.

Activists in Aleppo said the Free Syrian Army had swept into key Christian and Kurdish neighborhoods that were considered government strongholds. Fierce fighting was continuing in some of the areas, and the reports were confusing and difficult to verify.

Aleppo activist Amer Halabi said Kurdish fighters who had been aligned with the regime switched sides and joined rebels in an advance into the Kurdish neighborhood of Ashrafiyeh. Other reports said the fighters with the Kurdish PYD, or Democratic Union Party, had simply vacated their checkpoints overnight, enabling the rebels to overrun the area.

The rebels also advanced Thursday into the upscale Christian neighborhood of Siryan al-Jadidah but were encountering resistance from Syrian army units, said a resident who escaped to Beirut and asked not to be identified out of concerns for her safety. She said the usual daily shelling of the city had largely subsided, replaced by fierce hand-to-hand street fighting.

The advances appeared to put the rebels in control of most of the city center at a time when the government’s supply routes have been disrupted by recent opposition gains in the nearby province of Idlib.

Continued clashes were reported late into the night, and government artillery in the mountains overlooking Damascus also fired at least six shells into a southern suburb of the capital hours before the cease-fire was due to go into effect, according to the Britain-based Syrian Observatory for Human Rights.

‘Regime is not trustworthy’

The only other truce in the armed conflict that has raged throughout most of this year was in April, when government forces held the advantage over a newly formed and poorly armed rebel army. The cease-fire was short-lived, and the rebels have since dramatically expanded their capabilities. They now control large swaths of northern Syria.

“We are the ones attacking, and they are on defense,” Sheik told al-Jazeera, predicting that the government would break the truce and then try to blame the rebels. “The regime is not trustworthy.”

The rebels’ biggest fear is that the government will take advantage of the lull to reinforce its flagging positions in the north. But with the rebels on the advance and the government clearly under pressure, the cease-fire is of little benefit to the opposition, said Syria expert Joshua Landis, a history professor at the University of Oklahoma. He expressed doubt that it would succeed.

“I don’t think the rebels take it seriously, and I don’t think the Syrian government takes it seriously,” he said. But with the setbacks in Aleppo, “the negotiating power of the government has just fallen dramatically.”

Suzan Haidamous contributed from Beirut.