MOSCOW – A significant chapter in nuclear diplomacy closed on Thursday as the last standing nuclear arms agreement between Russia and the United States reached its expiration. This marks the first time in over fifty years that there are no restrictions on the world’s two largest nuclear arsenals.

The lapse of the New START Treaty has raised concerns about the potential for a renewed and unrestrained nuclear arms race between these global powers.

Last year, Russian President Vladimir Putin expressed willingness to adhere to the treaty’s constraints for an additional year, provided that the United States reciprocated. However, former U.S. President Donald Trump remained ambivalent about an extension, proposing instead that China should be included in any future agreement—a suggestion that China has consistently rejected.

On Wednesday, the expiration of the treaty was a topic of discussion between Putin and Chinese President Xi Jinping, according to Kremlin adviser Yuri Ushakov. Ushakov pointed out that Washington has yet to respond to Moscow’s offer to extend the treaty.

“Russia will proceed with caution and responsibility, evaluating the security landscape carefully,” Ushakov emphasized.

In a statement released on Wednesday night, Russia’s Foreign Ministry declared that, given the current situation, the parties to the New START Treaty are no longer bound by its obligations. This development effectively grants both nations the freedom to determine their future actions independently.

New START, signed in 2010 by then-President Barack Obama and his Russian counterpart, Dmitry Medvedev, restricted each side to no more than 1,550 nuclear warheads on no more than 700 missiles and bombers — deployed and ready for use. It was originally supposed to expire in 2021 but was extended for five more years.

The pact envisioned sweeping on-site inspections to verify compliance, although they stopped in 2020 because of the COVID-19 pandemic and never resumed.

In February 2023, Putin suspended Moscow’s participation, saying Russia couldn’t allow U.S. inspections of its nuclear sites at a time when Washington and its NATO allies have openly declared Moscow’s defeat in Ukraine as their goal. At the same time, the Kremlin emphasized it wasn’t withdrawing from the pact altogether, pledging to respect its caps on nuclear weapons.

In offering in September to abide by New START’s limits for a year to buy time for both sides to negotiate a successor agreement, Putin said the pact’s expiration would be destabilizing and could fuel nuclear proliferation.

New START followed a long succession of U.S.-Russian nuclear arms reduction pacts. Those have been terminated, as well.

Trump has indicated he would like to keep limits on nuclear weapons but wants to involve China in a potential new treaty.

“I actually feel strongly that if we’re going to do it, I think China should be a member of the extension,” Trump told The New York Times last month. “China should be a part of the agreement.”

Beijing has balked at any restrictions on its smaller but growing nuclear arsenal, while urging the U.S. to resume nuclear talks with Russia.

“China’s nuclear forces are not at all on the same scale as those of the U.S. and Russia, and thus China will not participate in nuclear disarmament negotiations at the current stage,” Foreign Ministry spokesperson Lin Jian said Thursday.

He said China regrets the expiration of the treaty, calls on the U.S. to resume nuclear dialogue with Russia soon and to positively respond to Moscow’s suggestion that the two sides continue observing the core limits of the treaty for now.

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This version of the story corrects the last paragraph to say China’s Foreign Ministry spokesperson Lin Jian said Beijing calls on the U.S. to respond positively to Moscow’s proposal to keep adhering to the treaty, not that China views it positively.

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Associated Press writer Ken Moritsugu contributed to this report from Beijing.