The Syrian army said it would suspend combat operations in the southern city of Deraa for 48 hours from Saturday, according to a statement carried by state news agency SANA.
A war monitor said the level of violence had fallen three hours after the ceasefire was due to take effect, but rebels said the city was still being bombarded.
The army general command said the ceasefire was due to take effect at 12 noon (0900 GMT) on Saturday and was being done to support “reconciliation efforts”.
The Syrian army and Iran-backed militia forces have escalated attacks against a rebel-held part of Deraa city in recent weeks, in a possible prelude to a large-scale campaign to wrest full control of the city.
But a rebel commander in Deraa told Reuters hostilities had not stopped.
“We have not heard of any such talk and the regime is still attacking us with the same intensity,” the commander said at 3:30 PM (1230 GMT).
The United States and Russia have been holding talks on creating a “de-escalation zone” in southwestern Syria which would include Deraa province, on the border with Jordan, and Quneitra, which borders the Israeli-occupied Golan Heights.
Deraa city lies within an existing plan brokered by Iran, Russia and Turkey in the Kazakh capital, Astana, in May to create four de-escalation zones in Syria. Since May, violence levels have vastly reduced in some of those proposed de-escalation areas, but fighting has continued on major frontline areas including in Deraa city.
The Syrian Observatory for Human Rights war monitor said there had been a decline in the pace of fighting and shelling in the city for the three hours since the ceasefire came into force. But the Britain-based monitor said some shells and air strikes had continued to hit parts of the city.
“There are breaches and we are distrustful of the regime’s intentions in abiding by the ceasefire,” Major Issam al Rayes, spokesman for the Southern Front grouping of the Free Syrian Army (FSA) rebels, told Reuters.
“The regimes forces have stopped their military operations after big losses in equipment and men since the start of their campaign over a month ago … after the failure of repeated attempts to advance,” Rayess said.