Syrian Observatory for Human Rights says fighting appears to have stopped
A ceasefire which has begun in Syria could be the last chance to work towards peace in the country, US Secretary of State John Kerry has said.
The Syrian army, which announced the truce at 5.00pm Irish time, said a "regime of calm" would be applied in all of Syria - and that it reserved the right to respond using firepower against any violation by armed groups.
Mr Kerry said it was too early to draw conclusions about how effective the stoppage would be, adding there was no doubt that violations would be reported "here and there".
Violence was reported in several areas in Syria just half an hour before the ceasefire was due to begin.
But after it took effect, the Syrian Observatory for Human Rights said fighting appeared to have stopped in major areas of conflict.
"Calm is prevailing," said director Rami Abdulrahman.
He added, though, that shelling had been reported by both rebel and government forces in the southwest.
The cessation of hostilities was brokered by the US and Russia - the second attempt this year by Washington and Moscow to bring the five-year civil war to an end.
Russia, meanwhile, has said "terrorists" will still be targeted.
The Russian Defence Ministry said it would continue air strikes against Islamic State and the former Nusra Front.
It also said a joint US-Russian centre was being set up to to coordinate coalition targets.
The ceasefire will be renewed every 48 hours. If it holds for a week, the US and Russia are due to begin sharing intelligence too.
It is also hoped that humanitarian supplies can be distributed.
In a statement, Russia's foreign ministry said aid would be delivered to the besieged city of Aleppo straight away.
Mikhail Bogdanov, Russia's deputy foreign minister, has said peace talks to end the civil war could begin again next month.
Earlier, President Bashar al-Assad was pictured praying at a mosque in a Damascus suburb recently taken from rebels.
Touring the district, he said he wanted to recapture the whole of Syria: "The Syrian state is determined to recover every area from the terrorists.
"The armed forces are continuing their work, relentlessly and without hesitation, regardless of internal or external circumstances," he said in Daraya.
Some rebel groups have expressed concern that the ceasefire is a "trap" which will largely benefit Bashar al Assad's government.
In a letter to the US, the Free Syrian Army said it planned to "cooperate positively" and respect the ceasefire, but wrote that a lack of enforcement mechanisms and a lack of provision for some of the country's most besieged areas were worrying.
The FSA also claimed that the exclusion of the Jabhat Fateh al Sham group - a former al Qaeda affiliate known as the Nusra Front - could be used by Russia as a pretext to bomb other rebel groups.
Reacting to its exclusion from the truce deal, a spokesman for Jabhat Fateh al Sham tweeted: "It's simple - the Russian-American deal is intended to eliminate those who protect Syrians."