WASHINGTON: The United States Friday urged supporters of the Syrian government to press it to "participate fully" in negotiations with the opposition, saying a lack of a political resolution in the war-torn country threatened indefinite instability.

In a statement, a spokeswoman for the State Department said the United States wanted the government's supporters "to use their leverage to urge the regime to participate fully in tangible negotiations with the opposition in Geneva."

"The United States urges all parties to work seriously toward a political resolution to this conflict or face continued isolation and instability indefinitely in Syria," spokeswoman Heather Nauert said in a statement.

A United Nations negotiator characterized a round of peace talks that ended on Thursday as a missed opportunity, and he laid most of the blame at the feet of Syria's government.