Frontlines across three provinces in southwest Syria have remained mostly quiet nearly 24 hours after an internationally brokered ceasefire took effect, according to a UK-based monitor.

The Syrian Observatory for Human Rights (SOHR) said there has been only sporadic activity in the provinces of Deraa, Suweida and Quneitra since the truce began at noon local time (09:00 GMT) on Sunday.

The United States, Russia and Jordan announced the agreement of the ceasefire earlier this week after a meeting between Russian President Vladimir Putin and his counterpart Donald Trump.

SOHR said the ceasefire appeared to be largely holding but reported sporadic incidents of violence.

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"There are minor violations that do not affect the ceasefire," SOHR director Rami Abdel Rahman said.

"In general there is quiet in the three provinces."

The monitor said two shells fired by Syrian government forces had landed on the town of Suweida in eastern Daraa overnight, while rebels and government forces had exchanged fire in the village of Al-Naeema in the same province.

Another two shells were fired by government forces into the Al-Balad area in the Daraa province and brief clashes erupted in the provincial capital Daraa city overnight but quickly stopped, SOHR added.

In Quneitra province, there were also reports of sporadic exchanges of fire, though there were no casualties in any of the incidents, the monitor said.

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Syria's government had already declared its own unilateral ceasefire in the area last week, but fighting had continued.

Sunday's ceasefire went into effect just ahead of new peace talks in Geneva, which begin later on Monday.

Similar agreements have been brokered in Syria in the past with the aim of getting the country's peace process back on track following a prolonged civil war that began in 2011.

All have failed to halt the fighting for very long.

More than 320,000 people have been killed in Syria since the conflict began in March 2011.