The U.N. special envoy negotiating with Syria's warring sides in Geneva says the parties have exchanged documents outlining their basic positions.

Staffan de Mistura says the documents can be used to find if there is common ground between the government's side and that of the Syrian opposition.

He said on Tuesday that each side "has to at least show" that it is "serious about wanting to find a political process or political transition."

The talks in Geneva are to adjourn later this week and continue in April.

This round of proximity negotiations in Switzerland, in which de Mistura is shuttling between the two sides, has offered more promise than previous attempts at negotiations amid a cease-fire that came into effect in late February and that has mostly held across Syria.