The cessation of hostilities in Syria that came into effect at sunset on Monday is holding well into its first day, reports suggest.
UK-based monitoring group the Syrian Observatory for Human Rights said it had recorded no civilian deaths in the first 15 hours of the truce.
Residents in the embattled northern city of Aleppo reported calm.
UN officials say they are waiting to deliver aid to besieged areas but need better guarantees of peace.
The fledgling truce was reportedly broken by sporadic attacks on Monday, carried out by both government forces and rebels after the ceasefire had come into effect.
The Syrian Observatory said they had seen reports of 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.
Humanitarian groups are hoping to make aid deliveries to the worst-hit areas, especially the war-torn city of Aleppo.
A UN spokesman said aid convoys were ready to travel in from Turkey but officials needed to see "a laying down of weapons that satisfies us... and as I speak that has not yet been the case".
, described on Friday by US Secretary of State John Kerry as the "last chance to save a united Syria", was struck on Friday in Geneva after months of talks between Russia and the US. It requires both sides to allow unhindered access for humanitarian aid.
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.
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".
Israel denied on Monday that Syria had shot down one of its jets over the Syrian Golan Heights.
Syrian state TV reported the countrys military had downed an Israeli warplane but the Israeli military responded with a statement saying that two Syrian surface-to-air missiles had missed its aircraft.
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-Assads 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...
groups like so-called Islamic State and Jabhat Fateh al-Sham face the joint might of the Russian and US air forces
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
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.