The Syrian government has declared a unilateral three-day ceasefire, set to last until the end of Friday, state-run news agency SANA reported.
The announcement coincides with the Eid al-Fitr holiday marking the end of the holy month of Ramadan.
The "regime of calm" had already entered into effect and would last until midnight on Friday, according to the report, which did not specify any geographic limits to the ceasefire.
The Britain-based Syrian Observatory for Human Rights said the truce will cover all Syrian territories, but was sceptical about the government's intentions.
Meanwhile, President Bashar al-Assad participated in holiday prayers at a mosque in Homs, a city in central Syria which has seen heavy battles during the civil war but is now under government control.
It was a rare public appearance for al-Assad outside the capital, Damascus.
State television broadcast pictures of al-Assad at the al-Safa mosque, in an area which has been frequently targeted in suicide attacks blamed on Islamic State and al-Qaeda's Syrian wing, al-Nusra Front.
The United States and Russia brokered a nationwide ceasefire in February which held for a time but then collapsed under the pressure of localized fighting and a failed political process to end the violence.
There have been attempts since then to reach short-term cessations of hostilities that are often region-specific, but these have tended to be short-lived.
The Observatory said that 1,138 Syrian civilians, including 249 children, were killed during Ramadan, which started on June 6.
Of these, 655 people, including 155 children, were killed by government forces and its Russian allies in airstrikes.
Rebel fighters and Islamic State killed 98 people in territory held by the government. The Islamic State group also executed 45 people.
Turkish border guards killed 17 people, the report said.