A U.S.-Russian brokered ceasefire for southwest Syria was holding hours after it took effect on Sunday, a monitor and two rebel officials said, in the latest international attempt at peace-making in the six-year war.
The United States, Russia and Jordan reached a ceasefire and "de-escalation agreement" this week with the aim of paving the way for a broader, more robust truce.
The announcement came after a meeting between U.S. President Donald Trump and Russian President Vladimir Putin at the G20 summit of major economies in Germany.
The Syrian Observatory for Human Rights, a Britain-based war monitoring group, said "calm was prevailing" with no air strikes or clashes in the southwest since the truce began at noon (0900 GMT) on Sunday.
"The situation is relatively calm," said Suhaib al-Ruhail, a spokesman for the Alwiyat al-Furqan rebel group in the Quneitra area.
Another rebel official, in Deraa city, said there had been no significant fighting. It was quiet on the main Manshiya front near the border with Jordan, which he said had been the site of some of the heaviest army bombing in recent weeks.
There was no immediate comment from the Syrian army.
A witness in Deraa said he had not seen warplanes in the sky or heard any fighting since noon.
Several ceasefires have crumbled since the onset of the conflict and it was not initially clear how much the combatants - Syrian government forces and the main rebels in the southwest - were committed to this latest effort.
With the help of Russian air power and Iranian-backed militias, Syrian President Bashar al-Assad's government has put rebels on the back foot over the last year. The wide array of mostly Sunni rebels include jihadist factions and other groups supported by Turkey, the United States and Gulf monarchies.
Earlier talks between the United States and Russia about a "de-escalation zone" in southwest Syria covered Deraa province on the border with Jordan, nearby Sweida and Quneitra which borders the Israeli-occupied Golan Heights.
A senior State Department official involved in the talks said further discussions would be necessary to decide crucial aspects of the agreement, including who will monitor its enforcement.
Russian Foreign Minister Sergei Lavrov said the deal includes "securing humanitarian access and setting up contacts between the opposition in the region and a monitoring centre that is being established in Jordan's capital."
The U.N. Deputy Special Envoy for Syria said on Saturday the deal was a "positive development" ahead of the latest round of U.N.-sponsored peace talks to begin in Geneva on Monday.
Western-backed rebels control swathes of Deraa and Quneitra, which are home to tens of thousands of people and form a centre of the insurgency south of the Syrian capital Damascus.
The multi-sided Syrian conflict, which grew out of popular protests against Assad's rule in 2011, has killed hundreds of thousands of people and created the world's worst refugee crisis.