The talks in Geneva open after a ceasefire took effect in three provinces in southern Syria on Sunday, with a monitor reporting that the region was mostly quiet despite scattered violations.

The ceasefire was brokered by the United States, Russia and Jordan, the latest agreement reached outside the Geneva framework.

In principle, the new round of Geneva negotiations will focus on four so-called "baskets": a new constitution, governance, elections and combating "terrorism".

As he arrived for the talks on Monday, UN Syria envoy Staffan de Mistura told reporters: "We will work very hard."

The last talks ended in May with little progress towards ending a war that has killed more than 320,000 people since it began in March 2011.

De Mistura said after that round that "important gaps remain... on major issues," and that time constraints had stymied progress.

Still, both sides are expected to participate once again, with Yehya al-Aridi, a spokesman for the opposition High Negotiations Committee, telling AFP he had "modest expectations".