BEIRUT — The embattled regime of Syrian President Bashar Assad promised Thursday to observe a U.N.-proposed truce during a four-day Muslim holiday, while rebels claimed major gains in the key battleground of Aleppo.

But prospects of the cease-fire taking hold are dim, given Assad's history of broken promises and the rebel momentum in Aleppo, Syria's largest city, where fighters said they advanced into several regime-held neighborhoods.

The truce plan by U.N.-Arab League envoy Lakhdar Brahimi has been endorsed by the U.N. Security Council, including Assad allies Russia and China. U.N. chief Ban Ki-moon urged all countries and groups with influence in Syria to pressure both sides to stop the violence in the civil war, his spokesman said.

The holiday cease-fire was the least a divided international community could agree on after the failure of a more ambitious plan for an open-ended truce and political transition talks by Brahimi's predecessor, Kofi Annan, in April.

Even the current truce, to begin Friday with the start of the Eid al-Adha holiday, appears in jeopardy from the outset. Neither side has shown an interest in laying down arms, instead pushing for incremental military gains.

The truce plan remained vague Thursday evening. It wasn't clear when exactly it was supposed to begin, and there were no arrangements for monitoring compliance. Brahimi never said what would happen after four days, a potentially dangerous omission considering that Assad and those trying to topple him sharply disagree on a way forward. Assad refuses to resign while the opposition says his departure is a prerequisite for talks.

"It's a longshot," Beirut-based analyst Paul Salem said of the cease-fire. "We are completely in war mode, at least for the next many months."

Both sides kept fighting into late Thursday.

In an apparent setback for the regime, activists said rebel fighters pushed into predominantly Christian and Kurdish neighborhoods in northern Aleppo that had previously been held by pro-Assad forces.

"It was a surprise," local activist Abu Raed said via Skype. "It was fast progress and in an unexpected direction."

He asked to be identified only by his nickname for fear of reprisals.

The battle for Aleppo, a former regime stronghold and Syria's business hub, has been largely deadlocked since rebels first captured parts of the city in late July. A complete rebel takeover could change the momentum of the war, although in recent months, front lines have shifted repeatedly and it was not clear if rebel fighters could maintain Thursday's gains.

Activists also reported fighting and shelling by government forces near the capital of Damascus, and scores of people were reported killed nationwide. Since the uprising against Assad began in March 2011, more than 35,000 people have been killed, including more than 8,000 government troops, according to activists.

Even as it lost some ground in Aleppo, the Assad regime said Thursday it would abide by the holiday truce. With Russia backing the truce and presumably bearing down on Damascus, such a step had been expected. Another Syria ally, Iran, welcomed what it called a "positive action" by Syria's army.

But in endorsing the plan, the Syrian military added major loopholes, saying it would respond with force not only if attacked, but if it believes opposition fighters are reinforcing positions or smuggling weapons from abroad.

The regime also accepted the previous cease-fire plan – proposed by Annan – which called for an open-ended truce to begin April 12. But it failed to implement major provisions, such as withdrawing troops and heavy weapons from urban centers. The truce soon collapsed.

Opposition leaders and rebel commanders dismissed Thursday's announcement by the regime as empty talk. Some said opposition fighters would halt their fire but respond if attacked by regime troops.

Gen. Mustafa al-Sheikh, a commander of the Free Syrian Army, said that "the brigades operating under the umbrella of this council will respect the cease-fire, if the regime indeed stops operations."

"However, we have experienced the regime's promises and lies before, ... Unfortunately with such dictatorial and sectarian regimes, you cannot believe such promises will be kept," he said.

The Syrian opposition is fractured and rebel fighters are organized in different groups, with rival agendas and command structures. Jabhat al-Nusra, a radical Islamic group that has been fighting alongside the rebels and has taken a lead in the battle for Aleppo, said it won't comply with the truce.

The U.S. put the onus on the Assad regime. "What we are hoping and expecting is that they will not just talk the talk of cease-fire, but that they will walk the walk, beginning with the regime," State Department spokeswoman Victoria Nuland said.

Iranian Foreign Minister Ali Akbar Salehi welcomed the cease-fire as a "positive action" in a telephone conversation with his Syrian counterpart, Walid al-Moallem, the semiofficial Fars news agency reported.

In Aleppo, it remained unclear how significant the rebels' gains were, as their forces often push into new areas only to swiftly abandon them when the regime bombs their positions.

An Aleppo activist reached via Skype said rebel fighters had seized the predominately Kurdish neighborhood of Ashrafiyeh and were pushing into Al-Siryan al-Jadideh, a nearby Christian neighborhood, where they were trying to capture a security office used as an army post.

The advance expanded the fight for Aleppo from the poorer, mostly Sunni Muslim neighborhoods on its eastern and southern sides – where the rebels can often count on support from the local population – into a new section of the city farther north.

The city's northwest has seen very little rebel activity since fighting in Aleppo began, and it was unclear how residents would react to the rebels, who are mostly from the countryside.

While the uprising has split Syria's Kurds between the rebels and the regime, the country's Christians have tried to remain neutral.

Abu Raed, the activist, said neither group had actively joined the uprising and that many were fleeing as the regime struck back.

"They have started leaving the neighborhoods because the shelling has started," he said.

Amateur video posted online Thursday showed gray smoke rising from a cluster of apartment buildings in Aleppo. A narrator said the video showed the aftermath of government shelling in the Midan neighborhood. In another video, a rebel fighter fired a machine gun from the back of a pickup truck before the vehicle sped off to take him out of the line of fire.

The videos appeared to be genuine, matching activist descriptions of events.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists, said more than two dozen people were killed in the city Thursday, including eight Kurds who died when mortar rounds exploded in their neighborhood. It was unclear who fired them, it said.

The Observatory also said about 20 people were killed in shelling and clashes near Damascus, most of them in the restive suburb of Duma.

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Associated Press writers Zeina Karam and Bassem Mroue in Beirut contributed reporting.

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