As leaders gathered in New York at the United Nations General Assembly, US Secretary of State John Kerry discussed Syria with Russian Foreign Minister Sergei Lavrov. He said that while it was vital to coordinate efforts against Islamic State militants this was not yet happening.
"I think we have concerns about how we are going to go forward," Kerry told reporters. US officials said Kerry was working on a new political initiative in New York that would include Russia and key regional powers.
A senior State Department official told reporters. "It was a very thorough exchange of views on both the military and the political implications of Russia's increased engagement in Syria."
Kerry also discussed Syria with Iran's foreign minister during a meeting at the United Nations on Saturday.
It was announced in Baghdad that Russian military officials were working with counterparts from Iran, Syria and Iraq on intelligence and security cooperation to counter Islamic State, which has captured large areas of both Syria and Iraq.
The move was seen in the region as potentially giving Moscow more sway in the Middle East.
Russian President Vladimir Putin derided US efforts to end the Syria war, which has driven a tide of refugees into neighboring states and Europe.
He said Moscow, which this month sent tanks and warplanes to a Russian military base in Syria, was itself trying to create a "coordinated framework" to resolve the conflict.
"We would welcome a common platform for collective action against the terrorists," Putin said in an interview on Sunday on CBS's "60 Minutes."
Putin, who will meet US President Barack Obama in New York today, branded US support for rebel forces in Syria as illegal and ineffective and said Damascus should be included in international efforts to fight Islamic State.
He mocked US plans to train up to 5,400 Syrian rebels to fight the group. "It turns out that only 60 of these fighters have been properly trained, and as few as four or five people actually carry weapons," he said.
Putin said Russia had no plans now to deploy combat troops. "Russia will not take part in any field operations on the territory of Syria or in other states; at least, we do not plan it for now," he said.
Referring to the risk of radicalised fighters returning home after fighting with Islamic State, he said: "There are more than 2,000 militants in Syria from the former Soviet Union. Instead of waiting for them to return back home we should help President al-Assad fight them there, in Syria."
Divisions over the role of Syrian President Bashar al-Assad remained critical.
President Hassan Rouhani of Iran, like Russia a big military supporter of Assad, told reporters any discussion of political reform in Syria should come only after the threat of "terrorism" had been removed.
The United States, Britain and some other allies in recent days have softened demands that Assad immediately leave power, raising the possibility that he could stay during a transition.
German Foreign Minister Frank-Walter Steinmeier, however, told ARD television on Sunday that the formation of a transitional Syrian government should be kept separate from discussions on the future of Assad.
Israel, which borders Syria and has previously attacked sites in Syria, carried out at least three air strikes against Syrian army targets on the Golan Heights on Sunday, rebel sources and a monitoring group said.