"Russia's compliance with the treaty has served our national security interests well, and Americans are much safer with New START intact and extended," Kirby said in a statement. "We cannot afford to lose New START's intrusive inspection and notification tools. Failing to swiftly extend New START would weaken America's understanding of Russia's long-range nuclear forces."
The five-year extension allows time and space for both countries to explore new verifiable arms control arrangements, Kirby said.
"Just as we engage Russia in ways that advance American interests, we in the Department will remain clear-eyed about the challenges Russia poses and committed to defending the nation against their reckless and adversarial actions," Kirby said.
Earlier Thursday, White House press secretary Jen Psaki told reporters the United States intended to seek a five-year extension of the agreement, but also that the White House has ordered an assessment of the massive cyberattack on federal agencies and departments related to the SolarWinds software breach, which analysts have blamed on Russia.
Earlier this month NATO Secretary General Jens Stoltenberg expressed concern about the pending expiration of the agreement, saying, "We need to make sure that we ... don't end up in a situation where there is no agreement regulating the number of nuclear warheads," Stoltenberg said.