US Secretary of State John Kerry is in Moscow for talks to try to bridge gaps with Russia over the political process to end Syria's civil war.
He is due to have meetings with President Vladimir Putin and Foreign Minister Sergei Lavrov.
The US and Russia have long disagreed on what role Syrian President Bashar al-Assad should play in the process.
The US wants Mr Assad to stand down but Russia says only the Syrian people can decide his fate.
Ahead of the talks, the Russian foreign ministry attacked US policy, accusing Washington of "dividing terrorists into good and bad ones".
Russia has been carrying out air strikes it says target Islamic State (IS) positions in Syria, but the US accuses Russia of bombing moderate rebels to shore up its ally President Assad. Moscow denies the allegations.
A US-led coalition has been targeting IS militants in Syria since September 2014 and does not co-ordinate its raids with the authorities in Damascus.
Russia has also rejected the outcome of a meeting of Syrian opposition groups, who agreed to unite to hold peace talks but stressed President Assad could not participate in a political transition.
Mr Kerry is to try to prepare the ground for an international meeting on Syria mooted for later this week.
There are some doubts whether it will go ahead but the US State Department denied claims from Russia "preconditions" would have to be met for the meeting to take place.
Negotiations between the Syrian government and the opposition have been tentatively scheduled for January next year.
Mr Kerry is also expected to use his Moscow talks for discussion of continuing efforts to restore stability in eastern Ukraine.
Buried under the avalanche of reactions to presidential candidate Donald Trump's verbal attack against Muslims, the stream of headlines about the San Bernardino couple who shot and killed more than 14 people last week and the dissection of President Barack Obama's Sunday Oval speech about the war against the so-called Islamic State (IS) was a very significant announcement.
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