WASHINGTON — International efforts to end the fighting in Syria have been dealt a serious blow, with the United States suspending direct contacts with Russia on halting the war, and chilly relations are turning even frostier after Russia put a hold on a plutonium disposal deal with Washington.

The two decisions, announced in their respective capitals just hours apart Monday, were ostensibly unrelated but underscored deep mistrust and rising tensions between the former Cold War foes, who are increasingly at odds on a number of issues, particularly Syria and Ukraine.

The moves further reduce areas of Washington-Moscow cooperation, yet their most immediate impact may be the potential death blow delivered to halting attempts to revive a moribund ceasefire in Syria, get desperately needed humanitarian aid to besieged communities and begin negotiations on political transition that could mean the ouster of President Bashar Assad.

The Obama administration said it decided to cut off discussions on Syria because Russia had not lived up to the terms of last month’s agreement to restore a tattered cease-fire and ensure sustained deliveries of humanitarian aid to besieged cities, such as Aleppo, which has been under bombardment from Russian and Syrian forces.

White House spokesman Josh Earnest said: “What’s clear is that there is nothing more for the United States and Russia to talk about with regard to trying to reach an agreement that would reduce the violence inside of Syria and that’s tragic.”

“This is not a decision that was taken lightly,” State Department spokesman John Kirby said in a statement. “Unfortunately, Russia failed to live up to its own commitments … and was also either unwilling or unable to ensure Syrian regime adherence to the arrangements to which Moscow agreed.”

Kirby’s statement said that Russia and Syria are pursuing military action in violation of the cease-fire agreement, and pointed to their targeting of hospitals as well as the Sept. 19 airstrike on a United Nations humanitarian aid convoy. The U.S. accused Russia of bombing that convoy, a charge both Russia and Syria have denied.

Russia intervened on behalf of its close ally Syria on Sept. 30 last year, joining Assad’s bombardment of both anti-Assad rebel groups and militant groups such as the Islamic State and Fatah al-Sham Front, an al-Qaida spinoff formerly known as the Nusra Front. Russia is interested in propping up Assad in part because Russia’s only naval facility outside the former Soviet Union is on the Syrian coast.

If it had been implemented, the cease-fire deal would have created a joint U.S.-Russian center to coordinate military and intelligence operations. President Barack Obama had overruled Pentagon objections to such cooperation and Secretary of State John Kerry made the offer.

According to a senior U.S. official, the Pentagon has ordered troops who had been deployed to set up the joint implementation center — fewer than 20 — to return to their bases. The official was not authorized to discuss the matter publicly, so the person spoke on condition of anonymity.

The suspension will not affect communications between the two countries aimed at keeping their planes from bumping into each other over Syria.

And, U.S. officials said that despite the suspension of talks with Russia, they would continue to work for a truce and aid deliveries to Syria in other gatherings, including the International Syria Support Group, a collection of nations that includes Russia. In Geneva, the U.N. special envoy for Syria Staffan de Mistura said he was disappointed by the suspension of talks but stressed the U.N. would continue to work on both the humanitarian aid and political fronts.

In Moscow, the Russian Foreign Ministry issued a statement, expressing “deep disappointment” about the U.S. move and blasting Washington for the failure to separate the rebels from al-Qaida’s branch in Syria.

“Washington’s decision reflects the inability of President Barack Obama’s administration to fulfill the key condition for the continuation of our cooperation in overcoming the Syrian crisis,” the statement said. “Or, perhaps, it never had an intention to do so. We are under a growing impression that in its striving for a much-desired change of power in Damascus Washington is ready to ‘make a deal with the Devil’ and forge a union with terrorists who want to turn history backwards and enforce their inhuman norms by force.”

The ministry added that “the stakes are high,” and warned that the “White House will bear the blame if Syria come under new blows by terrorists.”

The U.S. had agreed to separate the rebel groups but noted it was an extremely slow process. The U.S. has relatively few personnel on the ground in Syria and even the moderate rebels have said they are frustrated with the pace of U.S. help.

The suspension in Syria talks was announced just hours after the Kremlin said Russian President Vladimir Putin had signed a decree halting a joint program with the U.S. on the disposal of weapons grade plutonium.

The decree cited 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 cease-fire 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 expanded in 2006 and 2010, Russia and the U.S. each were to dispose of 34 metric tons of plutonium, enough material for about 17,000 nuclear warheads. When it was signed in 2000, the deal was touted as an example of successful cooperation between Washington and Moscow.

The Russian Foreign Ministry said the U.S. has “done all it could to destroy the atmosphere encouraging cooperation.” It cited U.S. sanctions on Moscow over its annexation of the Crimea region of Ukraine and NATO’s deployment of forces near Russian borders.

The White House and State Department voiced disappointment with Russia’s decision to opt out of the program.

Associated Press writers Vladimir Isachenkov in Moscow, Jamey Keaten in Geneva and Lolita C. Baldor in Washington contributed to this report.