A week-long ceasefire that took effect in Syria early this morning has reportedly been violated already.
Several Aleppo residents reported a government helicopter dropping explosive cylinders on a rebel-held district.
The US-Russian brokered ceasefire came into effect, with monitoring groups and state media reporting clashes up until the final minutes, and the most powerful rebel groups having yet to commit to the truce.
Syria's military announced that it would abide by a ceasefire until Sunday at midnight, while maintaining its right to defend itself against any violations.
The ceasefire marks the latest attempt to end the five-year conflict, which has killed more than 250,000 people and driven some 11 million from their homes.
The 2011 uprising against President Bashar Assad began with peaceful protests but escalated into a civil war following a brutal government crackdown and the rise of an armed insurgency.
Russia and Western nations hope the truce can lead to the revival of peace talks between Assad's government and the rebels battling to overthrow him, and contribute to efforts to defeat the Islamic State group and other extremists in Syria.
Russia's Deputy Foreign Minister Mikhail Bogdanov said peace talks between opposition groups and the government could resume as early as next month.
Ahmad al-Masalmeh, an opposition activist in the contested city of Daraa, said calm had prevailed over the city since 4 pm, but observers elsewhere in the country reported fighting all the way up to and after the start of the ceasefire.
In Aleppo, the northern city that has emerged as the epicentre of the fighting, opposition media activist Mahmoud Raslan said government helicopters dropped crude barrel bombs on a contested neighbourhood, while a doctor reported heavy shelling along the Castello road, a key route to besieged, opposition-held areas. He spoke on condition of anonymity for fear of retribution.
The terms of the agreement permit government forces to target the Al-Qaeda-linked Jabhat Fatah al-Sham for the first week of the ceasefire. It was unclear whether the group's positions were being targeted after the truce began.
Hours before the ceasefire went into effect, Assad vowed that his government would take back land from terrorists and rebuild the country.
In Geneva, the UN envoy for Syria said his office would monitor the start of the ceasefire.
The rebels and the Syrian government are expected to stop attacking one another. Assad's key allies _ Russia, Iran and the Lebanese militant group Hezbollah _ have also endorsed the deal.
Several previous negotiated ceasefires have all eventually collapsed.