Russia has warned of the risk of a “proxy war” in the Middle East after the US said it would send special forces to Syria.
Russian Foreign Minister Sergei Lavrov said this increased the need for co-operation between the US and Russia.
US officials said “fewer than 50” troops would “train, advise and assist” vetted opposition forces in fighting the so-called Islamic State (IS).
It will be the first time that US troops operate openly in Syria.
Lavrov said the US had decided on its move “unilaterally and without any reference to the Syrian leadership”.
He added: “I am convinced that neither the United States nor Russia of course want any kind of slide into a so-called proxy war But to me it is obvious that this situation makes the task of co-operation between the militaries even more relevant.”
He was speaking after talks with US Secretary of State John Kerry and UN envoy for Syria Staffan de Mistura in Vienna.
“Our role fundamentally and the strategy is to enable local forces but does that put US forces in harm’s way? It does, no question about it,” Defence Secretary Ash Carter later told reporters.
He did not rule out further deployments of special forces to the region, if the initial mission was deemed a success.
The US recently abandoned its Syria rebel training effort, opting to provide equipment and arms directly to rebel leaders instead, reports the BBC.
White House spokesman Josh Earnest said President Obama wanted to provide additional support for Syrian rebel fighters who had been having success on the battlefield.
“There are now moderate opposition forces that are 45 miles (72km) outside [IS stronghold] Raqqa,” he said. “The president is prepared to intensify the elements that have shown promise.”
He added: “This is an intensification of a strategy he discussed a year ago.”
This week’s talks in Vienna involved Iran for the first time.
The summit sought to close the gap between the US and its allies, who support the rebels, and the key foreign allies of the Syrian government, Russia and Iran.
World leaders say progress was made in the “historic” talks to resolve Syria’s civil war, but they continue to differ on the fate of President Bashar al-Assad.
