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.”