The U.N. special envoy for Syria says a resumption of peace talks between government envoys and representatives of the opposition is a "moment of truth" and insists the "only Plan B available is return to war."

Steffan de Mistura spoke to reporters Monday moments before resuming "proximity talks" in Geneva, a month after suspending them in an upsurge in violence in Syria. The talks come as he said a "fragile" cease-fire has largely held since Feb. 27, and humanitarian aid deliveries have resumed in recent weeks.

Many observers say the peace talks are the best chance in years to end a war moving into its sixth year, leaving at least 250,000 people dead and forcing at least 11 million people to leave their homes — many abroad.