Russia has test-fired two nuclear-capable cruise missiles, amid tensions between Moscow and Washington involving a proposed NATO missile defense shield.

Russian military officials say two long-range strategic bombers launched the missiles, and that the tests were successful.

Russia often steps up such activities at times of diplomatic tensions with the U.S.

Russia regards the defense missile shield as a security threat and has threatened to place missiles on NATO's borders if the alliance's plans go forward.

The U.S. and NATO say the shield does not target or threaten Russia in any way.

Former U.S. President George W. Bush proposed a land-based missile defense shield to protect NATO allies from a possible attack by Iran or other hostile governments. President Barack Obama later modified the plan to place some of the radar on ships.