The State Department just cut off its 'bilateral channels with Russia' over Syria

The US has formally suspended its bilateral channels with Russia over its continued aerial bombardment of Syria's largest city, Aleppo, State Department spokesman John Kirby confirmed in a statement released on Monday.

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

"The US spared no effort in negotiating and attempting to implement an arrangement with Russia aimed at reducing the violence, prividing 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 ceasefire 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 hmumanitarian aid from reaching civilians in need," the statement read.

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 over the past week in the worst bombings on the rebel-held eastern half of Aleppo since the war began in 2011.

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.

The cease-fire 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 has 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."

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