UN envoy says Syria talks at 'very critical moment'

"Definitely I am optimistic because it is the only way to be at such moments," Staffan de Mistura said. "It is a very, very critical moment."

He said a "full delegation of the opposition and a full delegation of the government" would be in the Austrian capital for the two days of talks.

France's foreign minister said meanwhile in Paris that the Vienna talks are the "last hope" for reaching a political solution to the seven-year war.

"There is no prospect of a political solution on Thursday except, and it's the last hope, the meeting that opens on Friday in Vienna led by the United Nations and with all the stakeholders present," Jean-Yves Le Drian said. Speaking in parliament, Le Drian also highlighted a "considerable worsening of the humanitarian situation" in Afrin, where Turkish forces are carrying out an offensive against a Kurdish militia, as well as in Idlib and in Eastern Ghouta.

The talks hosted by de Mistura in the Austrian capital come after eight previous rounds in Geneva that failed to get the warring parties even to talk to each other.