Russia-US relations fell to a new post-Cold War low 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" towards 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, has "done all it could to destroy the atmosphere encouraging cooperation".

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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 ceasefire deal negotiated last month.

The State Department announced that it was withdrawing US personnel who have been meeting in Geneva over the past several weeks with Russian counterparts to plan coordinated airstrikes against al-Qaeda and Isis terrorists in Syria. The coordination was to start as soon as a ceasefire, begun on September 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 year-long 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 US said it would work to separate US-backed opposition groups from terrorist forces.

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. Russia has justified its airstrikes by saying that the ceasefire - and US failure to disengage the opposition from the Front for the Conquest of Syria, the al-Qaeda 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".

Unrelenting strikes, many of them targeting hospitals and medical facilities, have "killed hundreds of children in a week's time," said the intelligence official.

"It's certainly the worst period between the two countries since the Cold War . . . it's obvious that things have been deteriorating across the board," said Philip Gordon of the Council on Foreign Relations.