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 the president to retake the entire country.
But less than an hour into the truce, violations were reported. Several 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.
There had been widespread doubts that the cease-fire, timed to coincide with the start of the Muslim holiday of Eid al-Adha, would be respected.
Under the terms, if it holds for seven days, the United States and Russia will undertake a new collaboration of airstrikes against jihadist militants in Syria, and the Syrian air force will be barred from flying over insurgent-held areas.
The United States supports an alliance of rebel groups and Russia supports President Bashar al-Assad, but both countries share an antipathy for the Islamic State and Nusra Front militants who have seized parts of the country.
Mr. Assad used the hours ahead of the cease-fire’s scheduled start to promise victory in his country’s five-year-old civil war, punctuating his pledge by visiting a Damascus suburb that rebels surrendered last month.
The visit by Mr. Assad to the suburb, Daraya, which had long been held by armed opposition fighters who want him deposed, was prominently reported by state television and other government news media.
The loss of Daraya, which once symbolized rebel defiance in the face of encirclement and relentless bombing attacks, reflected Mr. Assad’s strengthened position in the conflict since Russia intervened to help him a year ago.
An agreement on the cease-fire was reached late Friday by Russian and American diplomats meeting in Geneva, who have been struggling to find a way to reduce violence in the increasingly complex conflict so that food and medicine can reach civilians.
The agreement contains many caveats and there are widespread doubts that it will work, especially among the array of Syrian opposition groups backed by the United States, who fear that Mr. Assad is now even more entrenched in power.
Mr. Assad said nothing about the agreement in his appearance in Daraya reported by Syrian news media, which showed him praying at a mosque, walking past bombed buildings and driving his own silver sport utility vehicle.
“The Syrian state is determined to recover every area from the terrorists,” Mr. Assad said, using his blanket terminology for supporters of the insurgency that began in March 2011. He said Syria’s armed forces would prevail “regardless of any internal or external circumstances.”
There was no sign over the weekend that combatants in the conflict were putting down their weapons ahead of the cease-fire’s scheduled start. If anything, the fighting intensified.
Nearly 100 people were reported killed in attacks on rebel-held areas around the country on Saturday and Sunday, according to tallies by medical workers, rescuers and monitoring groups.