Paris: US Secretary of State John Kerry will head to Moscow on Tuesday on a delicate diplomatic mission to try to keep the fragile Syrian peace process on track.
In a sign of the complexity of the US-Russian relationship, the State Department said Kerry would meet President Vladimir Putin but the Kremlin would not confirm this.
"They will discuss ongoing efforts to achieve a political transition in Syria," US spokesman Mark Toner said on Friday on the sidelines of the UN climate summit in Paris.
But Kremlin spokesman Dmitry Peskov said a decision about whether Putin would meet Kerry could wait until after his talks with Foreign Minster Sergei Lavrov.
"John Kerry is coming here on the invitation of the Russian Ministry of Foreign Affairs. His partner in the talks will be Russia`s foreign minister," Peskov said.
"We do not exclude the possibility of such a meeting when Foreign Minister Sergei Lavrov and Secretary of State Kerry inform President Putin about their talks."
Kerry will also raise the issue of Ukraine, where Moscow stands accused of supporting pro-Russian separatist rebels, Toner said.
Russia will use the opportunity to protest the sanctions imposed on its economy by the US and its allies after it annexed Ukraine`s Crimea region.
The Russian foreign ministry said it hoped the visit, Kerry`s second this year, would improve what it said were the "complicated" relations between the rival countries.The talks will also cover differences in the parallel Russian and US approaches to the fight against the Islamic State group in Syria.
Washington accuses Moscow of using strikes against IS as cover for a campaign to shore up Bashar al-Assad`s regime against legitimate opposition movements.
The Kremlin insists its ally in Damascus has the sole right to authorise foreign military intervention against the jihadists, and that US bombing violates Syrian sovereignty.
State Department spokesman John Kirby said Friday that there was "still a lot of work to be done" in crafting an exit strategy for a war that has killed more than 250,000 people since March 2011.
"From the very beginning, there has been disparate views about the future of Assad, and that is not resolved today," he said.
Washington and Moscow are the key sponsors of the international bid to mediate an end to the Syrian war through the International Syrian Support Group.
The 17-nation group had been planning to meet in New York under UN auspices on December 18 to push forward plans for a negotiated ceasefire.
But the US and Russia are awaiting the results of a meeting between Syria`s splintered opposition before confirming the date.The rebels met Thursday in Riyadh and announced the composition of their team to open negotiations with Assad`s government.
But they also insisted that Assad must step down immediately at the start of the political transition process, which has a January 1 target date.
This insistence may be a sticking point for Assad`s allies in Moscow, and Kerry confirmed Friday he was working with the Saudis to iron out some problems with the rebel plan.
Russian Deputy Foreign Minister Gennady Gatilov meanwhile said he saw "prospects" for progress in the development of a transition plan, after meeting with representatives of the US and UN in Geneva on Friday.
The meeting "notably underlined the need to fix the most important problem of the day -- establishing a list of the opposition, the most representative possible, to participate in dialogue with the government, and a list of terrorist organisations that are not respecting the ceasefire," Gatilov was cited by the RIA Novosti state news agency as saying.
"Further progress of the process will depend on the speed with which these lists are compiled."
Russia has made it clear that it regards many of the rebel groups brought together under the Saudi initiative -- some of them hardline Islamists -- as "terrorists".