He said: "This is the best moment for trying the will of those who are committed." He added: "At this point there will be no victory. The question is how do you stabilize Syria. You cannot win a war anymore."

The day after John Kerry and Sergey Lavrov announced to put a cessation of hostilities in place in Syria and to expand the delivery of humanitarian aid to civilians, de Mistura told DW that next week will be crucial, when Russia and the USA will be chairing a special taskforce to prepare the implementation of the decisions.

The next round of peace talks "cannot be talks about talks," de Mistura said. "The Syrian people deserve to know that when we start talking, they will see the benefits on the ground."

On Russia he said: "Moscow is helping. They are part of the commitment. Moscow has influence on the government in Syria. Moscow has influence on some players, US on others." He expressed hope that the Syrian representatives would also be able to guarantee a cessation of hostilities.

De Mistura said he was "cautious because after five years we should not ever be not cautious. It’s a test but a positive test."

De Mistura said the six-hour negotiations between Saudi Arabia, Iran, Turkey, Qatar, Russia and the US on Thursday "are certainly helping me to feel comfortable relaunching the process."

He thanked Frank-Walter Steinmeier for his "strong support of what the UN is trying to do in this context." He also thanked John Kerry and Sergei Lavrov for cooperating "despite large differences still existing between the US and Russia."

De Mistura called the negotiations "pretty tough but worth it," asking: "What is the alternative? Going back to where we were before? No."