The Syrian army said on Monday it would suspend combat operations in southern Syria until Thursday to help a new round of peace talks in the Kazakh capital Astana.
The army general command said in a statement carried on state television the ceasefire began at midday (0900 GMT) on Sunday and was to support "reconciliation efforts", in the second such unilateral ceasefire in the last two weeks.
Russian-sponsored talks are planned in Astana on Tuesday. The last round of Astana talks in May led to the signing of an agreement between Iran, Russia and Turkey to create four de-escalation zones in Syria, one of which is in the south.
But fighting has continued in frontline areas, including in Deraa city in southern Syria, with hostilities expanding to the border province of Quneitra along the Israeli-occupied Golan Heights.
A rebel official said the latest ceasefire was a ploy to drag the opposition to Astana. The rebel side has already expressed deep misgivings about the de-escalation zones, which they say benefit the Syrian army by freeing forces to allow them to make territorial gains elsewhere.
"This ceasefire is an attempt by the Russians and the regime to bring back the opposition to Astana and give them assurances on the ground they will stop the shelling on condition they attend," said Sohaib Alraheel, spokesman of Liwa al Fuqan, a faction of the rebel Free Syrian Army (FSA) operating in southwest Syria.
A spokesman for the Southern Front, a coalition of Western-backed FSA rebel groups, cast doubt on whether the Syrian government army and its Iranian-backed allies would halt attacks on the front lines in Deraa and in Quneitra province.
"The Free Syrian Army are very distrustful of the regime's intentions in abiding by the ceasefire. It will be like the previous one," Major Issam al Rayes told Reuters.
The latest announcement for the first time extends the ceasefire to the whole of southern Syria, including the southwestern Quneitra province near the border with Israel and Sweida province in the southeast.
The United States and Russia have quietly held talks on creating a "de-escalation zone" in southern Syria, Western diplomats and regional officials said in early June.
The Israeli army has in recent days launched strikes against Syrian army outposts in the southwest, where the Iranian-backed Hezbollah has a strong presence, saying it was retaliating for errant fire from Syrian positions that landed in the Israeli-controlled Golan Heights.