MOSCOW (AP) — President Vladimir Putin on Monday suspended a deal with the United States on the disposal of weapons-grade plutonium, once a symbol of U.S.-Russian rapprochement that has fallen apart amid tensions over Ukraine, Syria and other disputes.

A strain in ties between the former Cold War rivals has escalated in recent weeks followed the collapse of a truce in Syria and the Syrian army’s massive onslaught in Aleppo under the cover of Russian warplanes.

Separately, the State Department said it was suspending bilateral contacts with Russia over Syria, following Secretary of State John Kerry’s threat to suspend contacts amid new attacks on the city of Aleppo.

Putin’s decree cited as reasons for Moscow’s move the “emerging threat to strategic stability as a result of U.S. unfriendly actions,” as well as Washington’s failure to meet its end of the deal. It said, however, that Russia will keep the weapons-grade plutonium covered under the agreement away from weapons programs.

Under the agreement, which was signed in 2000 and expanded in 2006 and 2010, Russia and the U.S. each were to dispose of 34 metric tons of weapons-grade plutonium, enough material for about 17,000 nuclear warheads.