BEIRUT, Lebanon — A cease-fire in the Syrian civil war, negotiated by Russia and the United States, officially took effect at sundown on Monday after a weekend of intensified fighting and a vow by Syria’s president to retake the entire country.

Despite pessimism over how long the cease-fire would last, calm was widely reported after it took effect at 7 p.m. local time, but there were a few notable exceptions.

Less than an hour into the truce, residents in the divided northern city of Aleppo said via text message that a government helicopter had dropped explosive cylinders on a rebel-held district. And in the southern province of Dara’a, a rebel faction said in a statement that it had killed four government soldiers. By midnight, opposition factions had reported 10 violations by government forces.

There have been extensive doubts expressed among many entangled in the conflict that the cease-fire, timed to coincide with the start of the Muslim holiday Eid al-Adha, will be respected.