NEWS BRIEF Russian President Vladimir Putin withdrew Monday from a post-Cold War agreement with the United States in which both countries agreed to get rid of plutonium that could be used in nuclear weapons.

Putin accused the U.S. for failing to stand by its side of the agreement, and for the heightened tension between the countries over the Syrian civil war.

The deal was originally signed in 2000 and renewed in 2009. Putin said he was now suspending cooperation because of “the emergence of a threat to strategic stability and as a result of unfriendly actions by the United States of America towards the Russian Federation.”

The 16-year-old agreement dealt with stockpiles of weapons-grade plutonium both countries could use to arm future nuclear warheads. Each country had agreed to destroy 34 tons of plutonium, enough to make 17,000 nuclear weapons. The U.S. had pushed for the deal as a means to end proliferation, and, in later years, out of the fear some of the plutonium could fall into the hands of terrorists. Hillary Clinton, at the time the secretary of state, helped finalize the deal back in 2009. Monday’s news shows just how far U.S.-Russian relations have deteriorated.

As Reuters reported, the Kremlin said:

"For quite a long time, Russia had been implementing it (the agreement) unilaterally," Kremlin spokesman Dmitry Peskov told a conference call with journalists on Monday.

"Now, taking into account this tension (in relations) in general ... the Russian side considers it impossible for the current state of things to last any longer."

Russia believes the U.S. hasn’t followed through with its side of the agreement because it changed the manner in which it will dispose the plutonium. As the Bulletin of Atomic Scientists pointed out, plutonium is hard to get rid of. Although it can’t be made completely unsuitable for use in weapons, it can be made difficult to recover and use. There are two primary ways to do this: One is to use plutonium to make mixed-oxide (MOX) fuel in nuclear reactors, a process that irradiates the plutonium and makes it extremely hard and costly to revert back into weapons-grade material; the other is known as immobilization, which does the same thing by mixing it with radioactive waste.

Russia is upset because the agreement had called for the first method of disposal, but President Obama this year—long after the deal was signed—proposed switching to the second method. This is because while Russia has kept on track with its MOX facility, the U.S. has not. Construction costs on the South Carolina facility meant to take the plutonium ballooned 20 percent, and utility companies have resisted using the fuel in their reactors.

The withdrawal undoubtedly also has to do with recent developments in Aleppo, Syria. The city has turned into a crucible for the five-year civil war as Russia and Syrian President Bashar al-Assad root out the last major rebel stronghold. Both Assad and Russia have recently been accused of bombing aid groups and civilians. In mid-September, the U.S. acknowledged it had bombed Syrian soldiers. These acts ended an agreed upon cease-fire, and have caused U.S.-Russian relations to deteriorate.

Part of Putin’s decree Monday assured the U.S. it had no intention of weaponizing the plutonium, though that assurance is no longer backed by bilateral agreement.