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.
A new "regime of calm" would begin from 1:00 a.m. on Saturday (22:00 GMT on Friday) and last one day in the capital´s eastern Ghouta suburb and three days in the northern countryside of the coastal province of Latakia, the army said in a statement.
But by excluding the city of Aleppo, scene of the worst recent violence, the narrow truces were unlikely to resurrect a ceasefire and peace talks that have collapsed this week.
It did not explain why the halt was only temporary, nor was there any mention of Aleppo, where a week of escalating violence has left more than 200 civilians dead.
State television said rebels unleashed a barrage of rocket fire into the government-held western part of Aleppo just as people were coming out of Friday prayers.
The Malla Khan mosque in Bab al-Faraj was hit, it said, causing heavy casualties. Some of the injured were in a serious condition and were likely to die, it added.
In the worst recent attack, an air strike destroyed a hospital in a rebel-held area overnight on Wednesday-Thursday. The French charity Medecins sans Frontieres, which supported the hospital, said on Friday the death toll had risen to at least 50, including six medics.
A Syrian military source said Aleppo was excluded from the newly announced truces "because in Aleppo there are terrorists who have not stopped hitting the city and its residents ... There are a large number of martyrs in Aleppo which is why the situation is different there".
Russia´s Interfax news agency quoted the officer in charge of a Russian ceasefire monitoring center as saying the truces meant all military action would cease in the covered areas.
Damascus described the truces as an attempt to salvage a wider "cessation of hostilities" agreement in place since February. That ceasefire, sponsored by Washington and Moscow, allowed peace talks to start but has all but completely collapsed in recent days along with the Geneva negotiations.
Violence was "soaring back to the levels we saw prior to the cessation of hostilities", said the U.N. human rights chief, Zeid Ra´ad al-Hussein.
"There are deeply disturbing reports of military build-ups indicating preparations for a lethal escalation," Zeid said in a statement that described a "monstrous disregard for civilian lives by all parties to the conflict".
The United Nations has called on Moscow and Washington to help restore the ceasefire to prevent the collapse of peace talks, which broke up this week in Geneva with virtually no progress after the opposition walked out.
"The cessation of hostilities and the Geneva talks were the only game in town, and if they are abandoned now, I dread to think how much more horror we will see in Syria," Zeid said.
Aleppo, Syria´s largest city before the war, has been divided for years between rebel and government zones. Full control would be the most important prize for President Bashar al-Assad, who has been fighting to keep hold of his country throughout a five-year civil war that U.N. mediator Staffan de Mistura says has killed up to 400,000 people.
Since Russia joined the war last year with air strikes against Assad´s enemies, battlefield momentum has shifted in the government´s favor.
Hundreds of thousands of people still live in rebel zones of Aleppo, and the countryside to the north includes the only stretch of the Turkish-Syrian border still in the hands of Assad´s main opponents, Arab Sunni Muslim rebel groups.
Opposition groups accuse the government of deliberately targeting civilians so they will abandon the area.