Fighting between government and rebel forces has been reported in parts of Syria despite a nationwide truce coming into force early on Friday.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said there had been fierce clashes and air strikes in northern Hama province.
It added that rebel-held Wadi Barada near Damascus was also bombarded. But the military denied doing so.
There has been no comment from Turkey and Russia, which brokered the truce.
The rival jihadist groups, Islamic State (IS) and Jabhat Fateh al-Sham, and the Kurdish YPG militia are excluded from the initiative, which is aimed at restarting peace talks in Kazakhstan's capital Astana next month.
Reports of clashes emerged within hours of the truce starting at midnight local time on Friday (22:00 GMT on Thursday).
The Syrian Observatory said government warplanes had carried out 16 air strikes on rebel-held areas in the northern countryside of Hama province during the day on Friday.
The Local Co-ordination Committees (LCC), an opposition activist network, said the town of Halfaya had been targeted.
The Syrian Observatory and LCC also reported fighting in Wadi Barada, a valley in the mountains north-west of Damascus.
They said helicopters had attacked the village of Basima and positions held by rebels and allied jihadists from Jabhat Fateh al-Sham, which was known as al-Nusra Front until it formally broke ties with al-Qaeda in July.
But a Syrian military media unit denied that troops had shelled Wadi Barada and accused the opposition of aiming to show it was not abiding by the truce.
The new kingmaker - Lyse Doucet, BBC chief international correspondent
This deal was declared before it was done and dusted. Seven groups said to have signed up include Ahrar al-Sham, which Moscow and Damascus have always described as terrorists. Ahrar al-Sham says it has "reservations". Do they have anything to do with backers like Saudi Arabia and Qatar?
But a new top table has been forged before a new US president enters the scene. Russia is confirmed as the foreign force which matters. Turkey displaced the US as kingmaker on the other side. It has bargaining chips and, most of all, wants to stop the sway of Syrian Kurdish forces, who are US allies.
Many opposition fighters will welcome a pause after their stinging defeat in Aleppo. But they and Turkey still want President Assad to step down. That conflicts with Iran, the other key player, as well as Mr Assad's own circles. But that's for the next round in this new great game which could be talks in Astana, in Russia's orbit.
The UN expressed concern about the fighting in Wadi Barada on Thursday, saying combatants were deliberately targeting and damaging springs used to supply some four million people in the Damascus area with drinking water.
The LCC also reported on Friday that government shellfire had caused casualties in rebel-held Douma, in the Eastern Ghouta region outside Damascus.
Abdulkafi Alhamdo, a teacher who was living in a rebel-held enclave in the northern city of Aleppo before being evacuated as part of a deal negotiated by Turkey and Russia earlier this month, said he was not optimistic.
"I can sleep a bit better and not wake up during the night in fear. But we have also experienced many ceasefires in the past and they don't last," he told the BBC.
"I believe [government forces] use that time to prepare their troops, and fix their planes and then they just target us again."
Meanwhile, Turkish military officials said Russian aircraft had carried out three air strikes against IS militants around the northern town of al-Bab.
The strikes appeared to be the first Russian support for a Turkish-backed rebel offensive aimed at recapturing the last IS stronghold in Aleppo province.
Russia has carried out an air campaign against President Bashar al-Assad's opponents since September 2015, while Turkey supports the rebellion.
Who is included by the truce agreement?
On the one side, Syrian government forces, allied militias and the Russian military.
On the other, a loose alliance of moderate rebel factions that operate under the banner of the Free Syrian Army (FSA), plus several other groups.
The Russian defence ministry named seven "moderate opposition formations" included in the truce as Faylaq al-Sham, Ahrar al-Sham, Jaysh al-Islam, Thuwwar Ahl al-Sham, Jaysh al-Mujahidin, Jaysh Idlib and Jabhah al-Shamiya.
Ahrar al-Sham, which said it had "reservations" about the deal, and Jaysh al-Islam are Islamist groups that Russia has previously described as terrorist organisations.
Who is not included?
The jihadist groups Islamic State and Jabhat Fateh al-Sham, "and the groups affiliated to them", are not part of the agreement, according to the Syrian army.
Jabhat Fateh al-Sham said on Friday it would continue to fight Bashar al-Assad, with a spokesman for the group saying the political solution under the truce would "reproduce the criminal regime".
Members of the group are currently operating as part of a rebel alliance that controls Idlib province.
The FSA also said the deal did not include the Kurdish Popular Protection Units (YPG). The militia, which has captured large swathes of north-eastern Syria from IS with US support, is designated a terrorist organisation by Turkey.
Where does it cover and what are the terms?
It is nominally nationwide, although that really only covers the areas where the sides who have signed up have a presence - western Syria.
Swathes of central and eastern Syria are under IS or Kurdish control.
Under the terms of the deal, talks on a political solution to end the civil war should begin within a month of the start of the truce and would be held in Kazakhstan.