A US-Russia-brokered ceasefire for southwest Syria is still in place hours after it took effect on Sunday, according to a monitor and two rebel officials.

The US, 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 to end the six-year war.

The announcement came after a meeting between the US president, Donald Trump, and his Russian counterpart 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 airstrikes or clashes in the southwest since the truce began at noon (9am 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 Daraa 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 Daraa 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 Iran-backed militias, the government of the Syrian president, Bashar al-Assad, has put rebels on the back foot over the last year. The wide array of mostly Sunni rebels include jihadi factions and other groups supported by Turkey, the US and Gulf monarchies.

Earlier talks between the US and Russia about a “de-escalation zone” in southwest Syria covered Daraa province on the border with Jordan, nearby Suwayda, and Quneitra, which borders the Israel-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.

Sergei Lavrov, the Russian foreign minister, said the deal included “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 UN deputy special envoy for Syria said on Saturday that the deal was a “positive development” before of the latest round of UN-sponsored peace talks, set to begin in Geneva on Monday.

Western-backed rebels control swaths of Daraa and Quneitra, which are home to tens of thousands of people and form a centre of the insurgency south of the Syrian capital Damascus.