The cessation of hostilities in Syria that came into effect at sunset on Monday appears to be holding, reports suggest.
The deal, brokered by Russia and the US has been described by the US Secretary of State, John Kerry, as possibly the “last chance to save a united Syria”.
Residents in the embattled northern city of Aleppo say there is calm there.
The Syrian Observatory for Human Rights monitoring group said it was “quiet” on nearly all fronts.
However, other reports spoke of sporadic attacks carried out by both government forces and rebels after the ceasefire had come into effect.
The Observatory said they included aerial bombardment of some villages in Hama province, and shelling near Damascus.
The Syrian army has said the truce will be applied throughout Syria for seven days, but that it reserves the right to respond decisively to any violation by armed groups.
A number of rebel factions have given a guarded welcome to the deal but expressed reservations about its implementation.
The deal was struck on Friday in Geneva after months of talks between Russia and the US. It also requires both sides to allow unhindered access for humanitarian aid to besieged areas.
Humanitarian groups are hoping to make aid deliveries to the worst-hit areas, especially the war-torn city of Aleppo.
If the truce holds for seven days, the US and Russia will carry out co-ordinated air strikes on militant groups – including so-called Islamic State and Jabhat Fateh al-Sham (known until recently as the Nusra Front).
The opposition Free Syrian Army group has said that while it will “co-operate positively” with the ceasefire, it was concerned it would benefit the government.
Media captionPresident Bashar al-Assad: “The Syrian state is determined to recover all areas from the terrorists”
Another major rebel group, the hardline Islamist Ahrar al-Sham, initially rejected the deal but later appeared to have softened its stance.
Opposition sources quoted by Reuters said a forthcoming statement supporting the cessation “with harsh reservations” would be backed by “the largest groups”, including Ahrar al-Sham.
Speaking earlier, President Bashar al-Assad welcomed the deal but said the Syrian state was still “determined to recover every area from the terrorists, and to rebuild”.
Big test for US and Russia: BBC Middle East editor Jeremy Bowen
The strength, or otherwise, of the ceasefire is a big test of what appears to be a less sour, more workable relationship between the foreign ministers of the US and Russia.
Diplomacy failed in the first, critical years of the war. A major reason for that was diplomatic deadlock between President Bashar al-Assad’s ally, Russia, and the US, which demanded his immediate departure from office.
Since then Russia has become the most influential outside power in Syria. The US and its Western allies have struggled to keep up.
Perhaps Moscow is now ready to build on a ceasefire, if it lasts, to push President Assad towards a political transition that might end the war.
Or perhaps, as enemies of President Assad and the Russians believe, the ceasefire will be a chance to regroup and rearm.
The conflict in Syria, which began with an uprising against Mr Assad, has raged for five years and claimed the lives of more than a quarter of a million people.
More than 4.8 million have fled abroad, and an estimated 6.5 million others have been displaced within the country, the UN says.
If the truce holds…
Jihadist groups like so-called Islamic State and Jabhat Fateh al-Sham face the joint might of the Russian and US air forces
Moderate rebels and civilians in the areas they hold will no longer face the threat of indiscriminate air strikes such as barrel-bombing although the Syrian air force will not be grounded completely; aid deliveries will be allowed to areas currently under siege
President Assad will be in a stronger position as the US and Russia engage two of his most effective military opponents while moderate rebels observe the truce with his forces
Syria’s history of failed deals
February 2012: Syrian government “categorically rejects” an Arab League plan calling for a joint Arab-UN peacekeeping mission
June 2012/January 2014/January 2016: Three failed UN-sponsored peace conferences in Geneva
September 2013: Kerry and Lavrov negotiate a deal to strip the Syrian government of its chemical weapons in return for the US backing away from air strikes. Since then, the government has again and repeatedly been accused of using toxic chemicals against rebel-held areas
February 2016: World powers agree in Munich on a nationwide “cessation of hostilities” in Syria excluding jihadist groups. There is no agreement on any joint US-Russian operations. The “pause” quickly unravels as Assad promises to regain control of the whole country
March 2016: President Vladimir Putin declares “mission accomplished” in Syria and orders removal of “main part” of Russia’s air army in Syria. Russian air strikes have continued ever since