Russian President Vladimir Putin reneged on a 2010 U.S.-Russia plutonium cleanup agreement, citing “unfriendly” behavior by the U.S. Sunday.

The agreement was developed in 2000 but not enforced until 2010, and signed by former Secretary of State Hillary Clinton and Russian Foreign Minister Sergei Lavrov. The agreement was seen as a step in the right direction by nuclear non-proliferation advocates.

The agreement created a framework for both countries to dispose of enough weapons-grade plutonium to make 17,000 nuclear weapons. Russia scrapping the program could enable both countries to launch into a new nuclear arms race.

“They see nuclear weapons as a normal extension of a conventional conflict,” Retired Army Gen. Philip Breedlove said of Russian strategic thinking to CBSNews. “I think to them the use of nuclear weapons is not unthinkable.”

Russia’s own military doctrine updated in 2014 says Russia “shall reserve the right to use nuclear weapons … In the event of aggression.”

The U.S. has responded by increasing the number of Cold War-era overflights in the North Pole. The exercises deploy nuclear capable bombers close to the Russian border to signal the ability of the U.S. nuclear arsenal.

In its rejection statement, Russia accused the U.S. of failure “to ensure the implementation of its obligations to utilize surplus weapons-grade plutonium.” Putin’s spokesman also claimed “Russia had been implementing it (the agreement) unilaterally.”

Putin’s rejection comes amid the lowest point in the U.S.-Russian relationship in decades. A U.S.-brokered ceasefire with Russia in Syria spectacularly collapsed in September, and the U.S. Ambassador to the United Nations openly accused Russia of perpetuating war crimes in Syria. The U.S. also renewed sanctions in September on Russia for its illegal annexation of Crimea, and surreptitious invasion of Ukraine.

Russia has a history of reneging on its nuclear agreements. The U.S. Department of State issued a statement in June 2014 alleging Russia had violated its nuclear agreement to not “possess, produce, or flight-test” an inter-continental ballistic missile. The State Department confirmed in 2015 that Russia remained in violation.

“Never having been made to pay a price, why wouldn’t Putin conclude that violations of the New START treaty would go unpunished as well?” Chairman of the House Armed Services Committee Mac Thornberry told the Free Beacon in June.

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