The decision to extend the pact, which expires on Feb. 5, was hailed by many arms control experts as important to stabilizing the relationship between the two largest nuclear-armed powers. But critics, including Trump's envoy for arms control who spent months negotiating with Russian officials, denounced it as a concession to Vladimir Putin.
"This extension makes more sense when the relationship with Russia is adversarial, as it is at this time," Psaki said during a briefing, calling the pact the "only remaining treaty constraining Russian nuclear forces" and "an anchor of strategic stability between our two countries."
The New Strategic Arms Reduction Treaty, a 2010 pact signed by Biden's former boss Barack Obama and known as New START, limits each side to 1,550 strategic nuclear warheads and includes verification measures like on-site inspections and data sharing.
Russia had been asking the U.S. to extend the treaty for five years -- a move allowed under the treaty's provisions. In a statement published while Biden was inaugurated Wednesday, the Russian Foreign Ministry criticized the Trump administration for ultimately refusing to extend the treaty and called for its immediate extension while the two sides negotiated a more expansive framework for nuclear arms control.
Trump's envoy for arms control Marshall Billingslea said the prior administration also sought that wider framework, but pursued a shorter-term freeze on both countries' nuclear weapons programs in the meantime, including caps on so-called "non-strategic" nuclear arms. Those are smaller-range or less advanced weapons, which are not covered under New START or other past nuclear arms-control treaties and of which Russia has a much larger stockpile.
"We are getting nothing for extending," tweeted Billingslea, accusing the Biden administration of "a stunning lack of negotiating skill."
Russia had rejected Billingslea's offer of a shorter-term extension or any freeze that included a verification regime.
With just two weeks until New START expires, arms control advocates welcomed Biden's decision, arguing it allows the administration to now use that five-year window to strike a larger deal and push to bring China into talks -- something Billingslea fought to do, but that Beijing repeatedly rebuffed.
Extension "sustains a stable U.S.-Russian nuclear balance and provides predictability as the U.S. modernizes its nuclear forces. Russia and the U.S. must stay within New START limits, avoiding an arms race," tweeted Rose Gottemoeller, the former deputy secretary-general of NATO and the top arms control official under the Obama administration who helped negotiate New START.
Predictability would be helpful, given the turmoil in the rest of U.S.-Russian relations, sinking even lower during Trump's term even as he called for "getting along" with Putin's government. Biden ordered the intelligence community to provide a full assessment of Russia's aggressive activity in the last year.
"Even as we work with Russia to advance U.S. interests, so too we work to hold Russia to account for its reckless and adversarial actions," Psaki said at the White House, intending to show a harder line on Russia early.
The assessments will cover the SolarWinds hack that affected dozens of government agencies and private companies, any interference in the 2020 elections, the use of chemical weapons attack to poison opposition leader Alexei Navalny, and the possible bounties offered the Taliban to killed U.S. troops in Afghanistan.
ABC News's Patrick Reevell contributed to this report from Moscow.