Ms Rice was seeking to counter the belief that a proposed US missile shield in Europe might threaten Russia's nuclear deterrent, a view she has suggested reflects a "hangover" from the long US-Soviet standoff.
Washington has angered Russia and unsettled some European allies with a plan to deploy 10 missile interceptors in Poland, and radar in the Czech Republic to help shield Europe from possible missile attack by nations such as Iran and North Korea.
"The idea that somehow 10 interceptors and a few radars in eastern Europe are going to threaten the Soviet strategic deterrent is purely ludicrous and everybody knows it," she told reporters in Oslo, where she is attending a NATO meeting.
Ms Rice said Washington wanted to keep discussing the issue with Moscow based on a "realistic" assessment rather than "one that is grounded somehow in the 1980s".
A Soviet specialist, Ms Rice served on the White House National Security Council from 1989 to March 1991, during the fall of the Berlin Wall, the collapse of communism in Europe and the waning days of the Soviet Union.