A ceasefire was holding across most of Syria on Friday but clashes near Damascus underlined the fragility of the deal brokered by rebel supporter Turkey and key regime ally Russia.
The nationwide truce, the first since September, is intended to pave the way for new peace talks in Kazakhstan being organised by Moscow, Ankara and Tehran.
The agreement comes a week after Syrian President Bashar al-Assad's army recaptured second city Aleppo in a major blow to rebel forces.
On the first day of the ceasefire Friday, the Britain-based Syrian Observatory for Human Rights reported sporadic violence in the Wadi Barada area, where rebels have cut water supplies to Damascus.
Observatory director Rami Abdel Rahman said helicopters carried out raids on rebel positions but it was unclear which side had started the clashes.
Syria's government had been shelling Wadi Barada before the truce began at midnight as it pushes rebels there to accept a "reconciliation deal" and leave the area.
The forces there include former Al-Qaeda affiliate Fateh al-Sham Front, previously known as Al-Nusra Front, which Syria's government says is excluded from the ceasefire.
Opposition figures however say the truce applies to all rebel-held territory, even where Fateh al-Sham is present.
Last week, rebels attacked water infrastructure in Wadi Barada and neighbouring Ain al-Fijeh, cutting supplies to the capital.
Four million people in Damascus and its suburbs have now been without water for a week, the UN says.
The clashes in Wadi Barada were the most serious of several isolated incidents of violence since the truce began.
The Observatory reported at least 16 government air strikes across several areas in Hama province in central Syria, but no casualties, but said a person was killed by regime sniper fire in the rebel bastion of Eastern Ghouta near Damascus.
- Tired of war -
In rebel-held Idlib province, however, it was quiet and residents expressed hope for respite from the bloody conflict.
"I support the ceasefire... and I support its continuation," said 31-year-old Ahmed Astify. "Everyone, whether (they are) rebels or regular people, is tired," he added.
Mohammed, 28, said: "We hope that this will lead to the end of the war."
Syria's government and its ally Iran both welcomed the ceasefire deal.
Damascus called it a "real opportunity" to find a political solution to the war, which has killed more than 310,000 people since it began with anti-regime protests in March 2011.
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Despite being left out of the process, Washington described the truce as "positive".
Analysts were cautious but said the involvement of Russia, Iran and Turkey could be important.
Sam Heller, fellow at The Century Foundation, said there was "real interest and urgency" from Moscow and Ankara, but expressed doubts about whether Tehran and Damascus were on board.
"All indications are that Iran and the regime want to continue towards a military conclusion," he said.
He said renewed fighting in Wadi Barada or Eastern Ghouta could pose major threats to the truce.
- Talks in Astana -
Russian President Vladimir Putin said Friday he would now reduce Moscow's military contingent in Syria, which has been fighting to bolster the government since last year.
But he added Russia would continue to fight "terrorism" and maintain its support for the government.
Turkey's President Recep Tayyip Erdogan also said Ankara would continue the operation it began in August targeting the Islamic State group and Kurdish fighters.
Moscow says seven key rebel groups have signed up to the deal, including the powerful Ahrar al-Sham, but the truce excludes jihadist organisations like IS or Fateh al-Sham.
But Syria's political opposition and rebels said the truce applied to all parts of the country.
"The agreement includes a ceasefire in all areas held by the moderate opposition, or by the moderate opposition and elements from Fateh al-Sham, such as Idlib province," said Ahmed Ramadan, a member of the National Coalition opposition body.
Despite backing opposite sides in the conflict, Turkey and Russia have worked increasingly closely on Syria, brokering a deal this month to allow the evacuation of tens of thousands of civilians and rebel fighters from Aleppo.
They are now pushing for peace talks between Damascus and the rebels to start next month in Kazakhstan's capital Astana.
UN peace envoy Staffan de Mistura said he hoped the agreement would "pave the way for productive talks", but also reiterated he wants negotiations mediated by his office to continue next year.
Russia, meanwhile, submitted a draft resolution to the UN Security Council supporting the ceasefire and the planned peace talks and was hoping for a unanimous vote on Saturday.
Moscow and Ankara say the Astana talks are meant to supplement UN-backed peace efforts, rather than replace them, and want to involve regional players like Egypt, Saudi Arabia, Qatar, and Jordan.
Washington is conspicuously absent from the new process, but Moscow said it hoped to bring US President-elect Donald Trump's administration on board once he takes office in January.