Russia says it has information that the United States planned to bomb the government quarter in Damascus on an invented pretext, and will respond militarily if it felt Russian lives were threatened by such an attack.
Moscow had information that rebels in the enclave of eastern Ghouta were planning to fake a chemical weapons attack against civilians and blame it on the Syrian army, Valery Gerasimov, head of Russia's General Staff, said.
He said the US intended to use the fake attack as a pretext to bomb the government quarter in nearby Damascus where Russian military advisers, Russian military police and Russian ceasefire monitors were based.
"In the event of a threat to the lives of our servicemen, Russia's armed forces will take retaliatory measures against the missiles and launchers used," Gerasimov said.
He did not say when the alleged attack would take place or provided detailed evidence to back his assertions.
In Washington, the Pentagon said Russia should focus on stopping Syrian President Bashar al-Assad from targeting innocent civilians.
"We urge Russia to stop creating distractions and compel the Assad regime to stop brutalising innocent Syrian citizens and allow much-needed aid to reach the people of East Ghouta and other remote areas," Pentagon spokesman Eric Pahon said.
During the Syrian army's offensive in eastern Ghouta, more than 1,100 civilians have died, the U.N. Office of Humanitarian Affairs said.
Assad's forces, backed by Russia and Iran, say they are targeting "terrorist" groups shelling the capital.
The head of Russia's military General Staff and chairman of the U.S. Joint Chiefs of Staff spoke by phone on Tuesday and discussed the situation in Syria.
Russia has previously accused rebels in Syria of preparing to use toxic agents in eastern Ghouta so they could later accuse Damascus of employing chemical weapons.