Russia, Turkey and Iran failed in talks on Wednesday to finalize an agreement on creating four de-escalation zones in Syria after Ankara raised objections, diplomats said.
Russia and Iran, which back President Bashar al-Assad's government, and Turkey, which supports some of the rebels, aim to reach a consensus on the zones by the end of August, when their delegations are set to meet again in the Kazakh capital.
The failure is a setback for Moscow, the main architect of the plan, as it seeks to take the lead in global efforts to settle the Syrian civil war.
"During these consultations, the Turkish side said it needed more time in order ... to make an appropriate decision," said senior Russian negotiator Alexander Lavrentyev.
Bashar al-Ja'afari, the lead negotiator from the Damascus government, was more direct.
"The Turkish delegation objected to the adoption of any documents related to the implementation of mechanisms of the agreement on the de-escalation zones," he said.
Russia, Turkey and Iran have set up a working group to reconcile their positions before the next Astana meeting set for the final week of August.
Lavrentyev said details related to the southern de-escalation zone needed to be agreed with the United States and Jordan, which support the rebels based there.
Turkish delegates made no statement to the media after the talks.
U.S. Secretary of State Rex Tillerson raised the possibility on Wednesday of several joint steps with Moscow to promote stability in Syria and urged all parties fighting Islamic State militants in the country to avoid conflict with one another.
"The United States is prepared to explore the possibility of establishing with Russia joint mechanisms for ensuring stability, including no-fly zones, on the ground ceasefire observers, and coordinated delivery of humanitarian assistance," Tillerson said in a statement issued by the State Department.
"If our two countries work together to establish stability on the ground, it will lay a foundation for progress on the settlement of Syria's political future," he said in the statement, which did not directly address the Astana talks.
The Kazakh talks, which began in January, come at a time when Turkey and Russia each want to disentangle themselves from the fighting.
That has led them into an ad hoc alliance, although they still trade barbs.
Since the May agreement was announced, the rebel-held stronghold of Idlib province in the northwest of Syria has been mostly calm.
But fighting has continued on other frontlines in western Syria, including Eastern Ghouta of Damascus and the southwestern city of Deraa, where government forces and their allies are trying to crush remaining pockets of rebellion.
If the agreement on de-escalation zones is finalised, Russia, Iran and Turkey will be able to quickly - within weeks - deploy forces such as military police on the borders of those zones, Lavrentyev has said.