A partial truce known as a “regime of calm” has come into effect in most parts of Syria apart from in the divided second city of Aleppo.
The truce announced by the Syrian army came into effect in two areas an hour after midnight local time on Friday.
A statement said it would last for 24 hours in Damascus and the Eastern Ghouta region outside the capital.
The truce is also due to last for 72 hours in the northern countryside of Latakia province.
There has been no explanation as to why the halt in fighting is only temporary,
During the day, rocket barrages fired by rebels were reported to have killed 15 worshippers after Friday prayers in a government-held part of Aleppo.
The Malla Khan mosque in Bab al-Faraj was hit, government officials said, causing heavy casualties. Some of the injured were in a serious condition and were likely to die, state television said.
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In the rebel-held east, government air strikes reportedly left at least 11 dead and destroyed a medical facility – the second to be hit in a week.
Crude barrel bombs were also reported to have smashed into residential neighbourhoods as rescue workers struggled to cope with the casualties.
The BBC’s Jim Muir says that the partial truce is the result of high-level contacts between the Americans and Russians, who are being urged by the UN and others to pull out all the stops to shore up the collapsing cessation of hostilities which came into effect two months ago.
Under the new arrangement, the Syrian army says the objective of the “regime of calm” is to deprive the rebels – or “terrorists” as it calls them – of a pretext for hitting civilian targets and to bolster the existing ceasefire.
The Americans are hoping that it will calm hostilities in those areas, and that the process can be extended to the biggest current trouble spot, Aleppo, which the Russians are reported to have refused to include in the latest truce.
More than 230 people are estimated to have been killed in the past eight days of deepening violence there.
Our correspondent say that has raised fears of a long and bitter new bout of hostilities, which would destroy any hope of a return to peace negotiations in Geneva.
Activists reported that government aircraft carried out the attacks on rebel-held parts of the city after hostilities eased for a short time overnight on Thursday.
The Syrian Civil Defence force, whose volunteer emergency response workers are known as the “white helmets”, said that among the sites targeted by warplanes was a clinic in the Marja area. A nurse was among several people hurt.
On Wednesday night, the al-Quds Hospital in the rebel-held Sukkari district was destroyed in air strikes that US Secretary of State John Kerry said appeared to have been “deliberate”.
Medicins Sans Frontieres (MSF), which supported the hospital, said on Friday that 50 people, including six members of staff, were now believed to have been killed.
The aid agency said the attack on al-Quds Hospital destroyed one of the last remaining places in Aleppo “in which you could still find humanity”. It has warned that government forces were preparing for a major offensive to encircle rebel-held areas of the city.
Large parts of the city have been destroyed and its infrastructure has been severely damaged, leaving civilians without water and electricity for months.
In an interview with the BBC, UN Secretary General Ban Ki-moon called on Syria’s leaders had to “go beyond their narrow personal perspectives” and call a halt to the destruction of their country.
The UN High Commissioner for Human Rights, Zeid Raad Al Hussein, said the failure of the Security Council to refer the situation in Syria to the International Criminal Court was “an example of the most shameful form of realpolitik”.
Russia and the US are backing opposing sides in the conflict.