Air strikes on Syria's northwestern Idlib region stopped on Friday after the government announced it had agreed to a truce following more than three months of deadly bombardment.
But militants dominating Idlib warned that regime violations would effectively "nullify" the ceasefire, which Damascus said will depend on rebel backer Turkey implementing a buffer zone in the area.
Most of Idlib province and parts of Hama, Aleppo and Latakia are controlled by Hayat Tahrir al-Sham, a jihadist group led by Syria's former Al-Qaeda affiliate.
The region is supposed to be protected from a massive government offensive by a Turkish-Russian deal struck in September in the Russian resort of Sochi.
But the deal has faltered and Syrian forces, along with Russia, have stepped up their bombardment there since the end of April, displacing 400,000 people according to the UN.
On Thursday, a new ceasefire went into effect as Syria peace talks resumed in Kazakhstan between Turkey and regime allies Russia and Iran.
"A cautious calm has reigned since just before midnight (2100 GMT)," Rami Abdel Rahman, head of the Britain-based Observatory for Human Rights told AFP on Friday.
He said that Syrian and Russia aircraft were no longer seen flying over Idlib following the truce welcomed by Moscow.
But regime forces and militants traded artillery fire in various parts of the region on Friday despite the deal, the Observatory said.
A civilian was killed and three others wounded in jihadist rocket fire near Qardaha -- the ancestral village of President Bashar al-Assad -- in Latakia, state news agency SANA said.
Since late April, some 790 civilians have been killed by regime and russian attacks, the Observatory says.
Fighting over the same period has claimed the lives of nearly two thousand combatants, including 900 regime loyalists, according to the monitor.
Syria's representative at the peace talks in Kazakhstan on Friday said the success of the latest truce depends on the implementation of last year's Turkish-Russian plan for a demilitarised buffer zone.
The September accord was never fully implemented as militants refused to withdraw from the area.
"The ceasefire agreement is conditioned on Turkey upholding the Astana and Sochi agreements," Bashar Jaafari said during the second day of talks in the Kazakh capital, calling the new truce "a test of Turkey's intentions".
He called on the guarantors of the talks to assume "their responsiblities by putting pressure on Turkey" to fulfil the conditions of last year's accord.
But he warned that Damascus would not be "waiting endlessly."
"Even though we are patient, this time our patience will be limited," he said.
HTS, however, said the success of the ceasefire would depend on Damascus.
"Any bombardment or assault on the cities and towns of the liberated north, will nullify the ceasefire agreement," it said in its first response to the truce.
In a statement obtained by AFP, HTS said it "reserves the right to respond" to any breach.
Assad, who now controls around 60 percent of the country, has vowed to reclaim the rest, including Idlib, Syria's last major jihadist bastion.
"I don't see the ceasefire lasting as Assad will not tolerate Idlib being outside his sphere of influence," said Syria researcher Samuel Ramani.
"But its a clever ploy likely instigated by Russia to bolster the credibility and perceived effectiveness of the Astana talks," he told AFP.
Abu Abdo, a 61-year-old from the Idlib town of Ariha, said the truce would be leveraged to secure regime gains.
"How many ceasefires have been brokered?" he asked rhetorically.
"They are cheaters. They have used every ceasefire to reclaim (opposition-held) areas," said the man who lost his wife and two children to bombardment on the region.
Abu Mohammad, 50, said he too was cynical.
"The regime has no credibility and we don't trust it at all," said the man, also from Ariha.
Syria expert Nawar Oliver said if anti-government groups in the region reject the truce Russia will continue "to bomb civilians and carry out massacres".
Or they could abide by the ceasefire, placing themselves at the mercy of Moscow "which has a track record violating agreements," Oliver told AFP.
The Syrian conflict has killed more than 370,000 people and driven millions from their homes since it started with the brutal repression of anti-government protests in 2011.