(CNN) More than 100 US troops began an unexpected exercise in southern Syria on Friday in direct response to a series of Russian military threats, according to a US defense official.

The exercise was detailed in a statement from the US Central Command but did not mention that the command ordered the exercise specifically after a September 1 warning from Russia that it was going to enter a restricted area of southern Syria where US troops are located, the official said.

The exercise, which may last for several days, involves US troops flying into the coalition's At Tanf garrison by assault helicopter and conducting live fire drills.

"Our forces will demonstrate the capability to deploy rapidly, assault a target with integrated air and ground forces and conduct rapid exfiltration," said Capt. William Urban, the chief Central Command spokesman. The exercise is specifically designed for Russian and regime units in the area to see the activity and understand US capabilities, defense officials say.

Russia on Thursday, via a note, had warned the US for the second time it was going to attack the At Tanf area to go after militants there. But the second warning specified Moscow would use "precision strikes," according to Cmdr. Sean Robertson, a Pentagon spokesman. The use of the wording "precision strikes" in a written note to the US sparked alarm with US commanders that the Russians are signaling their intent to strike with aircraft firing precision weapons, or missiles launched from Russian ships in the Mediterranean, US defense officials tell CNN.

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