The official told Fox News that Russian diplomats sent an official demarche ordering U.S. planes out of Syria, adding that Russian fighter jets were now flying over Syrian territory. U.S. military sources told Fox News that U.S. planes would not comply with the Russian demand.
"There is nothing to indicate that we are changing operations over Syria," a senior defense official said.
"We have had every indication in recent weeks that (the Russians) were going to do something given the build-up," another defense official added.
The move by Moscow marks a major escalation in ongoing tensions between the two countries over military action in the war-torn country and comes moments after Russian lawmakers formally approved a request from the country's president, Vladimir Putin, to authorize the use of troops in Syria.