Germany's top diplomat will host talks on Wednesday with his French counterpart, the UN's Syria envoy, and Syria's main opposition leader, as concerns grow over the war-torn country's faltering truce.

Discussions will focus on "how the conditions for a continuation of the peace talks in Geneva can be met, as well as how a reduction of violence and an improvement in the humanitarian situation in Syria can be achieved", the German foreign ministry said in a statement on Tuesday.