US Secretary of State John Kerry, left, and Russian Foreign Minister Sergei Lavrov at a joint news conference following their meeting in Moscow on July 16. REUTERS/Sergei Karpukhin

The US has formally suspended its negotiations with Russia over the cease-fire in Syria amid the continued Russian aerial bombardment of Syria's largest city, Aleppo, State Department spokesman John Kirby confirmed in a statement released Monday.

"This was not a decision that was taken lightly," the statement said.

"The US spared no effort in negotiating and attempting to implement an arrangement with Russia aimed at reducing the violence, providing unhindered humanitarian access, and degrading terrorist organizations operating in Syria.

"Unfortunately, Russia failed to live up to its own commitments," the statement continued. "Including its obligations under international humanitarian law and UNSCR 2254."

UNSCR 2254 is the UN Security Council Resolution adopted in December 2015 calling for a cease-fire and political settlement in Syria.

"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," the statement said.

Russian foreign ministry spokeswoman Maria Zakharova said the US is trying to shift blame onto Russia by suspending the talks, according to Reuters.

State-sponsored Russian news agency RIA Novosti reported that in response to the US move, Putin may order Russia's Duma to pass a bill that would allow Russia to maintain a military presence in Syria indefinitely.

U.S. Secretary of State Kerry and Russian Foreign Minister Lavrov shake hands during joint news conference following their meeting in Moscow Thomson Reuters

The formal suspension comes five days after US Secretary of State John Kerry first threatened to cut off the negotiations.

Kerry told his Russian counterpart Sergei Lavrov in a phone call last week that the US was preparing to "suspend US-Russia bilateral engagement on Syria ... unless Russia takes immediate steps to end the assault on Aleppo" and restore a cease-fire.

The State Department issued a statement about the call, saying "the secretary made clear the United States and its partners hold Russia responsible for this situation, including the use of incendiary and bunker buster bombs in an urban environment, a drastic escalation that puts civilians at great risk."

Hundreds of people have died during the past week in the worst bombings on the rebel-held eastern half of Aleppo since the war began in 2011.

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The bombings, which have also targeted rescue services in the city, punctuated the collapse of a fragile cease-fire brokered between the US and Russia earlier this month.

On Friday, Lavrov told the BBC that the US "still, in spite of many repeated promises and commitments... are not able or not willing to" separate the moderate opposition they support with former al Qaeda elements.

"We have more and more reasons to believe that from the very beginning the plan was to spare Nusra and to keep it just in case for Plan B or stage two when it would be time to change the regime," Lavrov said, referring to former al-Qaeda affiliate Jabhat al-Nusra. Nusra cut ties with the terror organization in August and rebranded itself as Jabhat Fateh al-Sham.

Deputy State Department spokesperson Mark Toner called the allegations "absurd."

A man walks on the rubble of damaged buildings after an airstrike on the rebel held al-Qaterji neighbourhood of Aleppo Thomson Reuters

The now-suspended bilateral channel was part of a deal between the US and Russia to coordinate their military operations in Syria and share intelligence about terrorist positions. That deal had been jeopardized by the latest scorched-earth offensive on Aleppo, however, with American and Russian diplomats exchanging diplomatic jabs early last week.

"What Russia is sponsoring and doing is not counterterrorism - it is barbarism," Samantha Power, the US ambassador to the United Nations, told member nations at a UN Security Council meeting last Sunday.

Kremlin spokesman Dmitry Peskov responded by calling the language "unacceptable."

A Joint Implementation Center was being set up in Jordan, where the US and Russia were set to coordinate their activities in Syria. But all personnel who had been dispatched to the center in anticipation of the US-Russia deal were going to be withdrawn.

"To ensure the safety of our respective military personnel and enable the fight against Daesh, the US will continue to utilize the channel of communications established with Russia to de-conflict counterterrorism operations in Syria," Kirby said.

The State Department's announcement came hours after Russian President Vladimir Putin suspended a treaty with Washington on cleaning up weapons-grade plutonium over the US's "unfriendly acts."

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