The UN Security Council scheduled a vote for Saturday on a resolution endorsing the ceasefire agreement in Syria and reiterating support for a roadmap to peace that starts with a transitional government.
Russia had been urging quick adoption of the resolution it sponsored, which also calls for "rapid, safe and unhindered" access to deliver humanitarian aid throughout the country. It calls, too, for a meeting in late January between the Syrian government and opposition in Kazakhstan's capital Astana "as an important part of the Syrian-led political process facilitated by the United Nations."
Russia's UN Ambassador, Vitaly Churkin, formally presented the draft at a closed council meeting Friday morning.
The agreement went into place midnight Friday and, if it holds, would mark a potential breakthrough in a conflict that began in 2011 with an uprising. However, monitors and rebels reported almost immediate clashes, and violence appeared to escalate later in the day as warplanes bombed areas in the country's northwest.
Syrian government warplanes carried out almost 20 raids against rebels in several towns along the provincial boundary between Idlib and Hama, the Syrian Observatory for Human Rights said.
Clashes between rebel groups and government forces took place overnight in the area, the Observatory and a rebel official said.
Warplanes and helicopters also struck northwest of Damascus in the rebel-held Wadi Barada valley, where government troops and allied forces clashed with rebels, the British-based Observatory reported.
A military media unit run by Damascus's ally Hezbollah denied any Syrian government air strikes on the area.
Opposition activist Mazen al-Shami, who is based in the Damascus suburb of Douma, said minor clashes nearby left one rebel wounded. Activist Ahmad al-Masalmeh, in the southern Daraa province, said government forces had opened fire on rebel-held areas.
An official from the Nour al-Din al-Zinki rebel group said government forces had also tried to advance in southern Aleppo province. There was no immediate comment from the Syrian military on Friday's clashes.
Several past attempts at halting the fighting have failed. As with previous agreements, the current ceasefire excludes both the al-Qaeda-affiliated Fatah al-Sham Front, which fights alongside other rebel factions, and the Islamic State in Iraq and Syria (ISIS).
The Union of Medical Care and Relief Organizations (UOSSM) has issued a statement saying it is "deeply concerned" about an ongoing water shortage in Damascus, which it says is affecting 70 per cent of the capital city's population.
The organization said the city's two main water sources — Wadi Barada and Ain-el-Fijah — were damaged in the fighting and have not been functional since Dec. 22.
"The lack of water and price gouging of commercially available water are forcing a large percentage of the population to drink substandard water. There is a significant risk of a water-borne disease pandemic — microbial parasites, typhoid, hepatitis and choleram," UOSSM said in the statement.
"The current medical infrastructure is not equipped to handle a pandemic of this scale; UOSSM urges the international community to immediately provide water, sanitation and hygiene aid to the people of Damascus.
Assad 'optimistic' about Trump
The truce came on the heels of a Russian-Turkish agreement earlier this month to evacuate the last rebels from eastern Aleppo after they were confined to a tiny enclave by a government offensive. The retaking of all of Aleppo marked Assad's greatest victory since the start of the 2011 uprising against his family's four-decade rule.
"The defeat of the terrorists in Aleppo is an important step toward ending the war," Assad said in an interview with TG5, an Italian TV station, adding that the capture of the city does not mean that the war has ended because "terrorists" are still in Syria.
The United States was left out of both agreements, reflecting the deterioration of relations between Moscow and Washington after the failure of previous diplomatic efforts on Syria.
Assad told TG5 "we are more optimistic, with caution," about the incoming administration of U.S. president-elect Donald Trump, who has suggested greater cooperation with Russia against extremist groups.
ISIS militants killed, reports Turkey
Turkey's state-run Anadolu Agency, meanwhile, quoted the military as saying Russia carried out three airstrikes against ISIS targets near the northern town of al-Bab, where Turkish troops and allied Syrian opposition forces have been battling the group.
The Turkish military statement quoted by Anadolu did not say when the Russian airstrikes took place, but said they killed 12 ISIS militants.
Separately, 26 ISIS militants, including some senior commanders, were killed in Turkish airstrikes on al-Bab and the Daglabash region, and some 17 ISIS targets were destroyed, Anadolu reported. It said a Turkish soldier was killed in an ISIS attack on troops south of the al-Azrak area.
Turkey sent troops and tanks into northern Syria in August to help opposition forces clear a border area of ISIS militants and curb the advances of U.S.-backed Syrian Kurdish fighters, who are also battling the group.