UPDATED 12.15pm: A Syrian rebel commander said that government shelling had stopped in some parts of Syria but continued elsewhere in what he described as a violation of a cessation of hostilities agreement.
The ceasefire agreement entered into force at midnight under a U.S.-Russia plan which the United Nations has called the best hope for peace in the five-year conflict.
Fares Bayoush, head of the Fursan al-Haqq rebel group which fights under the banner of the Free Syrian Army, told Reuters that continued violations could lead to a collapse in the agreement.
"There are areas where the bombardment has stopped but there are areas where there are violations by the regime such as Kafr Zeita in Hama, via targeting with artillery, and likewise in Morek in northern Hama countryside," he said, adding that FSA groups remain committed to the truce.
Bayoush said it was normal for violations to take place in any truce but the other side should halt them if it was serious about the agreement.
"But if these violations continue they may lead to the collapse of the agreement," he said.
The temporary truce does not apply to Islamic State and al Qaeda affiliate the Nusra Front, and the Syrian government and Moscow have said they will not halt combat against those militants. Other rebels seen as moderates by the West say they fear this will be used to justify attacks on them.
Nusra Front, one of Syria's most powerful Islamist rebel groups, yesterday urged insurgents to intensify their attacks on President Bashar al-Assad and his allies, adding to dangers facing the agreement.
Fighting raged across much of western Syria right up until the cessation came into effect, the Britain-based Syrian Observatory for Human Rights said.
There was calm in many parts of the country shortly after midnight, it said.
"In Damascus and its countryside ... for the first time in years, calm prevails," Observatory director Rami Abdulrahman said.
"In Latakia, calm, and at the Hmeimim air base there is no plane activity," he said, referring to the Latakia base where Russia's warplanes operate.
The United Nations unanimously demanded late last night that all parties to the conflict comply with terms of the plan as part of efforts to end a war that has killed more than 250,000 people and driven 11 million from their homes. De Mistura said he intends to restart peace talks on March 7, provided the halt in fighting largely holds.
On Friday, at least 40 government soldiers and allied fighters and 18 insurgents were killed in battles and air strikes in Latakia province, the Observatory reported.
Six people were also killed in an air raid in western Aleppo province in the hours before the halt, it said.
Near Damascus, dozens of air raids hit the besieged suburb of Daraya. Rescue workers said at least five people were killed in Douma, northeast of the capital.
Russian President Vladimir Putin stressed on Friday that combat actions against Islamic State, the Nusra Front and other groups the Syrian government regards as terrorists would continue.
The United States said it was time for Russia to show it was serious about halting fighting by honoring a commitment not to strike Syrian groups that are part of the moderate opposition.