The United Nations Syria envoy on Thursday hailed the work done at the Astana meeting by the three guarantor countries, Iran, Russia and Turkey, to ensure that "the Idlib de-escalation arrangements are sustained."

In a statement, Staffan de Mistura recognized "the initial, although still very limited, movement on the issue of detainees."

However, de Mistura lamented that the three Sochi co-conveners "produced no tangible results in overcoming the 10-month stalemate on the composition of the constitutional committee."

He said the last Astana meeting of 2018 was "a missed opportunity to accelerate the establishment of a credible, balanced and inclusive, Syrian-owned, Syrian-led, U.N.-facilitated constitutional committee."

The U.N. special envoy pointed out that the three guarantor countries committed to intensify their efforts to facilitate further efforts next week. According to the statement, de Mistura would continue working for the establishment of a constitutional committee before Dec. 31.

On Oct. 17, de Mistura announced that he will leave his post at the end of November citing "personal reasons." Veteran Norwegian diplomat Geir Pedersen has been named as his successor.

Meanwhile, following de Mistura's statements, the U.S. said on Thursday that the Astana process has only led to a "stalemate" in efforts to establish a constitutional committee crucial to a political settlement. 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.

"Russia and Iran continue to use the process to mask the Bashar Assad regime's refusal to engage in the political process" under U.N. 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 11th round of Syria peace talks in the Astana format concluded on Thursday with a decision to step up joint efforts to prevent violations of the cease-fire in Idlib.

In the joint statement following the 11th round of talks, it was stressed that the parties "examined in details the situation in the Idlib de-escalation area and reaffirmed their determination to fully implement the Memorandum on the Stabilization of the Situation in the Idlib de-escalation Area from Sept. 17, 2018."

"In this regard, they expressed their concern with the ongoing violations of the cease-fire regime and declared that, as guarantors of the cease-fire regime, they would step up their efforts to ensure observations, including by enhancing work of the Joint Iranian-Russian-Turkish Coordination Center," the statement added.

Russia, Iran and Turkey launched the Astana peace process in the Kazakh capital of Astana in January 2017 as a complementary part of the U.N.-sponsored peace talks in Geneva. It brought all warring parties in the Syrian conflict to the table to find a political solution.

The Sochi agreement was reached on Sept. 17 by President Recep Tayyip Erdoğan and his Russian counterpart Vladimir Putin. The deal established a cease-fire in the Idlib region, the last opposition stronghold in Syria, on the condition that heavy arms and extremist groups would withdraw from the region.

Prior to the agreement, the Assad regime was signaling an expansive military operation in Idlib, sparking fears in the international community of a new humanitarian crisis. As there have been some violations of the Sochi deal, Turkey called on all parties to restore calm and warned against provocations.