A nationwide ceasefire brokered by the United States and Russia has come into effect in Syria, the second attempt this year to halt the five-year conflict.
The truce, brokered by the United States and Russia, entered into force at 7pm local time or 4pm Irish time, across Syria except in jihadist-held areas.
Rebel groups fighting to topple President Bashar al-Assad had decided to respect the ceasefire, while expressing their deep reservations about the overall agreement, said Zakaria Malahifji of the Aleppo-based rebel faction Fastaqim.
Combatant sources on both sides said calm was prevailing in the first hours of the ceasefire.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, shared that assessment.
Russia is a major backer of Mr Assad, while the US supports some of the rebel groups fighting to topple him.
US Secretary of State John Kerry said early reports suggested there had been some reduction in violence.
He told reporters at the State Department that it was too early to draw a definitive conclusion about how effective the truce will be, and that there would no doubt be some reports of violations "here and there".
The agreement's initial aims include allowing humanitarian access and joint US-Russian targeting of jihadist groups, which are not covered by the agreement.
Russia's Foreign Ministry said in a statement that aid to the besieged city of Aleppo would start immediately.
The agreement comes at a time when Mr Assad's position on the battlefield is stronger than it has been since the earliest months of the war, thanks to Russian and Iranian military support.
Hundreds of thousands of Syrians have been killed in the conflict and 11 million made homeless in the world's worst refugee crisis.
Hours before the truce took effect, an emboldened Mr Assad vowed to take back all of Syria.
In a gesture loaded with symbolism, state television showed him visiting Daraya, a Damascus suburb long held by rebels but recaptured last month after fighters surrendered in the face of a crushing siege.
"The Syrian state is determined to recover every area from the terrorists," he said in an interview broadcast by state media.
Earlier he performed Muslim holiday prayers alongside other officials in a bare hall in a Daraya mosque.
He made no mention of the ceasefire agreement, but said the army would continue its work "without hesitation, regardless of any internal or external circumstances".
The ceasefire is the boldest expression yet of hope by the administration of US President Barack Obama that it can work with Russian President Vladimir Putin to end the war.
All previous diplomatic initiatives have collapsed in failure.
The Obama administration opposes the Assad regime but wants to shift the focus of fighting from the multi-sided civil war between Mr Assad and his many foes to a campaign against so-called Islamic State, an ultra-hardline jihadist group that controls swathes of Syria and neighbouring Iraq.
The ceasefire deal is backed by foreign countries ranging from Mr Assad's ally Iran to Turkey, one of the main supporters of groups fighting to overthrow him.
But maintaining the ceasefire means overcoming big challenges, including separating nationalist rebels who would be protected under it from jihadist fighters who are excluded.
The rebels say the deal benefits Mr Assad, whose military position has improved since the last truce brokered by Washington and Moscow collapsed earlier this year.
The capture of Daraya, a few kilometres from Damascus, has helped the government secure important areas to the southwest of the capital near an air base.
The army has also completely encircled the rebel-held half of Aleppo, Syria's largest city before the war, which has been divided into government and opposition-held zones for years.
Russia said the truce would cover the entire country but Moscow would continue to strike "terrorist targets".
If the truce holds for one week, the United States and Russia will then work together to develop military strikes against jihadists such as the Islamic State group and former Al-Qaeda ally Fateh al-Sham.
IS spokesman al-Adnani confirmed killed in US air strike: Pentagon
The Pentagon has confirmed that a US air strike killed Islamic State leader and spokesman Abu Mohamed al-Adnani in northern Syria last month.
"The strike near Al Bab, Syria, removes from the battlefield ISIL's chief propagandist, recruiter and architect of external terrorist operations," Pentagon press secretary Peter Cook said, using an acronym for the Islamic State group.
"It is one in a series of successful strikes against ISIL leaders, including those responsible for finances and military planning, that make it harder for the group to operate."
The 39 August air strike was conducted by a Predator drone, which fired a Hellfire missile at the car Adnani was traveling in.
Officials say Adnani was the main spokesman for IS, and he had played a major role during some of the group's most high-profile attacks over the past year, including in Paris, at the Brussels and Istanbul airports, at a cafe in Bangladesh, as well as the downing of a Russian airliner in the Sinai and suicide bombings at a rally in Ankara.
