The Syrian army said Wednesday it was observing a 72-hour ceasefire across the country for the holidays marking the end of the Islamic holy month of Ramadan.
The announcement did not say if the truce extends to jihadists of the Islamic State (IS) group or Al-Qaeda-affiliated Al-Nusra Front, but a security source said they were apparently excluded.
As fighting continued on the ground, especially in second city Aleppo, some rebel groups said they would respect the truce although they doubted the seriousness of President Bashar al-Assad's regime.
The Kremlin said later that Russian President Vladimir Putin and US counterpart Barack Obama agreed in a telephone call to "intensify" military cooperation in the war-torn country.
"The two parties have confirmed their desire to intensify coordination between the Russian and American militaries in Syria," it said without elaborating.
The statement came after US Secretary of State John Kerry hailed the ceasefire and said he was working with Russia and others to try to transform it into a lasting truce.
Syria's conflict began in 2011 with the brutal repression of anti-government demonstrations and has evolved into a complex multi-front war.
The IS emerged from the chaos, seizing large parts of Syria and Iraq, committing widespread atrocities, and organising and inspiring attacks across the Middle East and in Western cities.
Washington has backed rebel forces in Syria and Moscow is supporting Assad, but the rise of IS has seen efforts focus on defeating the jihadists.
- 'Regime of silence' -
Attempts to implement ceasefires in Syria have repeatedly failed, as have diplomatic efforts to end a five-year civil war that has killed more than 280,000 people and forced millions from their homes.
Russia and the United States launched a major attempt last year to bring about peace talks between Assad and rebel forces, but the negotiations faltered as the partial truce announced in February fell apart.
The latest takes effect during the three-day Eid al-Fitr festival marking the end of Ramadan, when devout Muslims fast from dawn until dusk.
"A 'regime of silence' is applied across all territory of the Syrian Arab Republic for 72 hours from 1:00 am on July 6 to midnight on July 8," the army said as it announced the move.
Assad attended Eid prayers in third city Homs, in a rare public appearance outside the capital.
State television showed him joining worshippers at the Al-Safa mosque in Homs, which is mainly under government control except for a besieged neighbourhood on the city's outskirts.
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In Tbilisi, Kerry welcomed the "Syrian army's declaration of a period of quiet in honour and celebration of Eid".
"So is 72 hours enough? The answer is simple: 'no'. Is 72 hours more welcome than nothing? The answer is 'yes'."
A ceasefire announced by Russia and the United States in late February has mostly collapsed amid repeated violations.
Temporary truces have also been announced in recent months for Aleppo, scene of some of the heaviest fighting in the war, but have fallen apart.
- Scepticism -
Aleppo, Syria's pre-war commercial hub, is divided between a rebel-held east and regime-controlled west.
Eastern districts have faced heavy bombardment including regime air strikes, while rebels regularly fire rockets into the west.
On Wednesday, Aleppo's eastern areas came under fire, said an AFP correspondent and the Syrian Observatory for Human Rights.
In the east's Al-Mashad district, a civilian was killed and several wounded when artillery fire fell near a mosque holding Eid prayers, the Observatory said.
Rebel forces fired back at regime-held areas of the city with mortars, the monitor said.
"Shells have been falling on the neighbourhood since early this morning," said Al-Mashad resident Ahmad Naseef.
"I had planned to visit family and friends, to bring my children out to play, but we decided to stay inside the house in case the shelling resumes," the 30-year-old told AFP.
"I hope things will calm down a bit during Eid, not for me but for my children."
In a statement, several rebel groups announced they would observe the truce "as long as it is respected by the other side".
"Up until now, it has not lived up to its announcement, as it launched several attacks in several regions today," they said.
In Damascus, residents were sceptical.
"I don't believe in this truce; earlier ones were violated repeatedly," said Saad al-Sawwas, 25.