Russia will withdraw from the original pact and subsequent amendments, the decree says, meaning that the country will no longer be treaty-bound to destroy its plutonium stockpiles. But the decree also offers an assurance, backed by no bilateral agreement, that the plutonium will not be used for military purposes.

“These agreements were designed to limit and circumscribe the future chances of getting back into a competition over nuclear arms,” James Collins, an associate at the Carnegie Endowment for International Peace, said in a telephone interview. “It was an important step in defusing the strategic nuclear arms race.”

Mr. Collins, who was the United States’ ambassador to Russia when the agreement was signed, called the abrogation a “strange move,” given the extraordinary danger, not least to Russians, should plutonium fall into terrorist hands. He added that it was “in my understanding the first time they have withdrawn from a specific nuclear agreement,” highlighting the slide in relations lately.

Russia and the United States had reaffirmed the plutonium disposal agreement in 2009, as President Obama pursued the “reset” policy with Dmitri A. Medvedev, then the Russian president.

Russia had viewed the agreement as rendering disarmament irreversible by destroying the fissile materials accumulated during the Cold War. In this light, the Russians had interpreted the treaty as requiring that the plutonium be irreversibly transformed into nonexplosive materials by using it in civilian nuclear power plants as a type of fuel, called mixed oxide fuel, or mox. Russia is pressing ahead with that.

But glitches and cost overruns in the mox plant at Savannah River, S.C., delayed the American program. This year, Mr. Obama proposed canceling the program in the 2017 budget and instead sending the plutonium for long-term storage at a nuclear waste site in Carlsbad, N.M.

The State Department has said the move complies with the treaty, but the Russians have said it does not, as Mr. Putin reaffirmed on Monday.