US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov on Saturday discussed who should represent the Syrian opposition at crucial talks with the government as time ticks down on the meeting scheduled for next week.
The talks -- part of the biggest diplomatic push yet to resolve a civil war which began almost five years ago -- were scheduled to open in Geneva on Monday.
But the United Nations has said they may be delayed several days amid intense wrangling over the makeup of the opposition delegation.
Kerry called Lavrov to discuss the talks following a face-to-face meeting on Wednesday, the Russian foreign ministry said in a statement, with both men backing efforts to get them started in Geneva "next week".
"Particular attention was given to the need to form a genuinely representative opposition delegation," the statement said.
The US chief diplomat is currently in Saudi Arabia and is scheduled to meet with top Syrian opposition officials.
Diplomats appear to be scrambling to resolve the key question of who will represent Syria's fractious opposition at the talks with the regime of Bashar al-Assad.
Russia -- which is flying a bombing campaign in Syria in support of Assad's forces -- has disagreed with the United States and its regional allies over which groups should be allowed to take part.
Last week the main coalition of opposition bodies, the High Negotiations Committee, announced a delegation to the talks on Wednesday -- but its composition quickly came under fire.
The group named Mohamed Alloush, a political leader of the Saudi-backed Islamist armed group Jaish al-Islam, as its chief negotiator at the talks.
But Syria's ally Moscow has said it considers Jaish al-Islam as "terrorists" and other opposition groups insisted it was unacceptable for the delegation to be led by a member of an armed group.
The High Negotiations Committee says it must be the sole opposition representative, but other opposition figures and Syria's Kurds have been excluded from its ranks and also want to participate in the talks.