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WASHINGTON — In a sharp deterioration of relations, the U.S. on Monday suspended diplomatic contacts with Russia over Syria, while Moscow halted cooperation on a joint program for disposal of weapons-grade plutonium.

The U.S. move followed a threat last week from Secretary of State John Kerry after new Russian and Syrian attacks on the city of Aleppo. The State Department said Russia had not lived up to the terms of an agreement last month to restore the cease-fire and ensure sustained deliveries of humanitarian aid to besieged cities.

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"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."

Russian President Valdimir Putin and U.S. President Barack Obama shake hands for the cameras before the start of a bilateral meeting at the United Nations headquarters September 28, 2015 in New York City. Chip Somodevilla / Getty Images

"Rather, Russia and the Syrian regime have chosen to pursue a military course, inconsistent with the Cessation of Hostilities, as demonstrated by their intensified attacks against civilian areas, targeting of critical infrastructure such as hospitals, and preventing humanitarian aid from reaching civilians in need, including through the September 19 attack on a humanitarian aid convoy," he said.

An airstrike last month hit a United Nations humanitarian aid convoy, killing 20 people. The United States has accused Russia of hitting the convoy, but both Russia and Syria deny it.

Monday's announcement came just hours after Russian President Vladimir Putin suspended a Russia-U.S. deal on the disposal of weapons-grade plutonium, in a move that also underscored rising tensions between Washington and Moscow.

Putin's decree cited Washington's "unfriendly actions" and the United States' inability to fulfill its obligations under the 2000 deal as reasons for the move. Under the agreement, which was 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.

White House spokesman Josh Earnest said patience had run out with Russia.

"What is clear is that there is nothing more for the United States and Russia to talk about with regard to stopping the ongoing violence in Syria and that is unfortunate," he told reporters.

He said the U.S. would withdraw personnel that it had dispatched to take part in the creation of a joint U.S.-Russia center that was to have coordinated military cooperation and intelligence had the cease-fire taken hold. The suspension will not affect communications between the two countries aimed at de-conflicting counter-terrorism operations in Syria.

Last week, amid the deteriorating conditions, Kerry threatened to suspend contacts with Russia unless "immediate" action was taken to ease the situation. Despite no improvements, however, he did not order the suspension until Monday.