U.S.-Russia relations fell to a new post-Cold War low Monday as the Obama administration abandoned efforts to cooperate with Russia on ending the Syrian civil war and forming a common front against terrorists there, and Moscow suspended a landmark nuclear agreement.

The latter move, scuttling a deal the two countries signed in 2000 to dispose of their stocks of weapons-grade plutonium, was largely symbolic. But it provided the Kremlin with an opportunity to cite a series of what it called “unfriendly actions” toward Russia – from Ukraine-related and human rights sanctions to the deployment of NATO forces in the Baltics.

The United States, the Russian foreign ministry said in a statement, has “done all it could to destroy the atmosphere encouraging cooperation.”

Of far more immediate concern, the end of the Syria deal left the administration with no apparent diplomatic options remaining to stop the carnage in Aleppo and beyond after the collapse of a short-lived cease-fire deal negotiated last month.

The State Department announced that it was withdrawing U.S. personnel who have been meeting in Geneva over the past several weeks with Russian counterparts to plan coordinated airstrikes against al-Qaida and Islamic State terrorists in Syria. The coordination was to start as soon as a cease-fire, begun Sept. 12, took hold and humanitarian aid began to flow to besieged communities where civilians have borne the brunt of Russian-backed President Bashar al-Assad’s response to a five-year effort to oust him.

The Syria agreement was part of a yearlong effort spearheaded by Secretary of State John Kerry to persuade Russia to help bring a negotiated political end to the war. In exchange for using its leverage with Assad to ground his air force and allow aid to flow, the United States said it would work to separate U.S.-backed opposition groups from terrorist forces with which they have become increasingly intertwined.

Instead, after just a few days of a fitful truce, both Syria and Russia stepped up their bombing attacks in Aleppo and elsewhere in the country, including the destruction of an aid convoy. Russia, while denying the convoy attack, has justified its airstrikes by saying that the cease-fire – along with U.S. failure to disengage the opposition from the Front for the Conquest of Syria, the al-Qaida group formerly known as Jabhat al-Nusra – allowed the terrorists to rearm and expand their territory.

Russia’s version of the sequence of events mandated by the deal is “explicitly not true,” a senior administration official said. “Separation was not step one,” but was supposed to occur after seven days without major violence. The Russians, the official said, have “constantly tried to move the goal posts.”

The Damascus government officially declared the truce over on Sept. 19. Since then, Syrian and Russian aircraft have carried out a massive barrage from the air over Aleppo.

Unrelenting strikes, many of them targeting hospitals and medical facilities, have “killed hundreds of children in a week’s time,” said the intelligence official, one of several who spoke on the condition of anonymity under administration-imposed rules to discuss the Syrian conflict.

Kerry said last week that the United States was “on the verge” of suspending the Syria agreement. “The decision today came after a long, deliberate period of trying to give them every benefit of the doubt,” the senior administration official said. “But when they’re targeting hospitals, and using these kinds of weapons, that’s not a misunderstanding.”