Establishment and convening of the committee by year's end "is vital to a lasting de-escalation and a political solution to the conflict," State Department spokeswoman Heather Nauert said in a statement.

Her comments came after the outgoing United Nations envoy to Syria, Staffan de Mistura, regretted that there was "no tangible progress" on the composition of the constitutional committee at two days of talks which ended Thursday in the Kazakh capital Astana.

Moscow and Tehran, allies of the Damascus regime, began the Astana process in January 2017 along with rebel-backer Turkey.

The Astana process followed a Russian military intervention which tipped the military balance in favor of Syrian President Bashar al-Assad's authoritarian regime.

"Russia and Iran continue to use the process to mask the Assad regime's refusal to engage in the political process" under UN auspices, Nauert said.

She added that "success is not possible without the international community holding Damascus fully accountable for the lack of progress in resolving the conflict."

The Astana process has gradually eclipsed the earlier UN-sponsored negotiations framework known as the Geneva process, which had put more emphasis on political transition but failed to curb violence that has killed more than 360,000 people and displaced millions.

Syria's war began in March 2011 as an uprising against Assad but morphed into a complex conflict with myriad armed groups, many of whom are foreign-backed.