The start of the talks in Geneva coincided with the first full day of a cease- fire for southern Syria that was brokered last week by the US, Russia and Jordan.
The Britain-based Syrian Observatory for Human Rights monitoring group on Monday reported fighting in the Sweida province, near the boundaries of the cease-fire zone.
However, opposition activist Ahmad al-Masalmeh said it was quiet in the city of Daraa, which has scene fierce clashes and which is located at the heart of the cease-fire zone.
The agreed-on cease-fire covers three war-torn provinces in southern Syria. It's the first tangible outcome following months of strategy and diplomacy between the new Trump administration and Russian President Vladimir Putin's Moscow.
The Geneva talks are expected to last through the week. UN envoy Staffan de Mistura declined to comment on his expectations but was due to hold a news conference later in the day.
It comes as Iran said the ceasefire should be expanded to all of Syria.
"The (ceasefire) agreement can be fruitful if it is expanded to all of Syria and includes all the area that we discussed in Astana talks for de-escalating the tension," Iranian foreign ministry spokesman Bahram Qasemi was quoted as saying by Tasnim news agency.
"Iran is seeking Syria's sovereignty and security so a ceasefire cannot be limited to a certain location ... No agreement would be successful without taking the realities on the ground into account," he added.
