Syria war: Cessation of hostilities comes into effect
A cessation of hostilities has come into effect in Syria, although it is unclear how widely it will be observed.
The Syrian army says it is implementing the truce, which began at sunset, but rebel groups have been more guarded.
US Secretary of State John Kerry, who helped broker the deal, warned it could be the last chance for peace in a united Syria.
Humanitarian groups are hoping to make aid deliveries to the worst-hit areas, especially the war-torn city of Aleppo.
Mr Kerry, speaking at the state department in Washington, said early reports indicated "some reduction in violence".
But he said that it was too early to draw a definitive conclusion about how effective the truce would be.
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Just after the ceasefire came into effect at sunset on Monday, the Syrian army announced a seven-day "freeze" on military operations.
The Syrian Observatory for Human Rights monitoring group reported that calm appeared to be prevailing on most front lines.
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.
If the truce holds for seven days, the US and Russia will carry out co-ordinated air strikes on militant groups.
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".
The cessation of violence is due to be renewed every 48 hours.
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 truce followed a weekend of air strikes by government forces on several rebel areas that killed about 100 people.
Russian warplanes were also in action in the provinces of Idlib and Aleppo, say Syrian activists.
Such intensification of violence has occurred before other, aborted, ceasefires in Syria.
Under the plan, Syrian government forces will halt combat missions in specified opposition-held areas.
Russia and the US will then establish a joint centre to combat jihadist groups, including so-called Islamic State and Jabhat Fateh al-Sham (known until recently as the Nusra Front).
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