More On Russia

"The key issues that have divided Moscow and Washington are a long way from being resolved."

The S-300 has a range of up to 200 kilometers (125 miles); the S-400 up to 400 kilometers (250 miles).

The head of U.S. Central Command had few kind words Thursday for Russian forces operating in Syria.

Russia has amassed as many as 40,000 troops and dozens of fighter jets, helicopters, tanks and armored vehicles near Ukraine.

The Air Force needs the help of the Army to outsmart Russia's enhanced S-400 anti-air defense system, a top general said.

The U.S. shouldn't play catch up to Russia or China in tech development, according to the U.S. Air Force's chief scientist.