US Secretary of State John Kerry said he wanted to use a visit to Moscow on Tuesday to make "real progress" in narrowing differences with Russian leader Vladimir Putin over how to end the conflict in Syria.
Kerry is seeking to prepare the ground for a third round of talks of world powers on Syria but it was not clear if a meeting penciled in for Friday in New York would go ahead.
Russia and the United States have not reached agreement over the role of Syrian President Bashar al-Assad in any political transition or over which rebel groups should be part of talks.
"I look forward to making real progress," Kerry said at the start of talks with Russian Foreign Minister Sergei Lavrov.
"I think the world benefits when powerful nations with a long history with each other have the ability to be able to find common ground," he said.
Kerry, who will meet Putin later on Tuesday, told Lavrov: "Even when there have been differences between us we have been able to work effectively on specific issues."
Russia, one of Assad's staunchest allies, has launched a campaign of air strikes that it said targeted Islamic State militants but which also supported Assad's forces. The Kremlin says the Syrian people, and not external powers, should decide Assad's political fate.
Lavrov told Kerry there was a need for more effective international cooperation in fighting terror. "On that route there are still questions which today we need to look at," Lavrov said in his opening remarks.
The run-up to the Moscow talks underlined the distance between Moscow and Washington on how to deal with the Syria crisis.
Speaking before Kerry's arrival, a State Department official said Kerry would raise concerns about Russia's continued bombing of Syrian opposition forces, including some backed by Washington and its allies.
For its part, the Russian Foreign Ministry issued a statement complaining that Washington was not ready to fully cooperate in the struggle against Islamic State militants and needed to rethink its policy of "dividing terrorists into good and bad ones".
Kerry's visit follows a meeting last week in Riyadh which agreed to unite a number of opposition groups, not including Islamic State, to negotiate with Damascus in peace talks.
While Kerry said there were still "kinks" that needed to be worked out, mainly to do with which groups should be included in the talks, the Kremlin rejected the results of the Riyadh meeting, saying some of the groups were considered terrorists.
No deal on Ukraine
"We don't have a full meeting of the minds yet (on Assad)," a senior State Department official told reporters. "We will talk about some of the details of a transition…in the hopes of narrowing the differences between us."
Kerry's talks in Moscow will delve deeper into details of a planned January 1 ceasefire in Syria, as well as Monday's comments by Russia on supporting the Free Syrian Army (FSA), which has been fighting Russian- and Iranian-backed government forces.
"We'll be interested to hear what the Russians have in mind there, given the Free Syrian Army's concern about how Assad has been treating his own people," the official said.
Russia has given mixed messages on the FSA. News agencies on Monday quoted Valery Gerasimov, the chief of the Russian army's general staff, as saying his country was providing it with weapons, ammunition and material support, while a top Kremlin aide later said there was no such arrangement.
FSA rebels fighting Assad's forces in western Syria denied receiving any support from the Russian air force, saying on the contrary it continued to bomb them.
White House spokesman Josh Earnest said Kerry will encourage continued efforts by Russia to de-escalate tensions with Turkey after it shot down a Russian military plane near the Syria border on November 24.
Kerry will also underscore the importance of implementing the Minsk peace accords on Ukraine and plans for free and fair elections in eastern Ukraine.