BEIRUT — The embattled regime of Syrian President Bashar Assad promised on Thursday to observe a UN-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 UN-Arab League envoy Lakhdar Brahimi has been endorsed by the UN Security Council, including Assad allies Russia and China. UN Secretary General 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 was not 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.
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.’’
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.
General 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.