The US holds Russia responsible for the violence in Aleppo and is threatening to break off all cooperation with Moscow in Syria, the State Department said. Russia is asking the US to live up to its obligation to separate US-backed opposition from terrorists.

In a phone conversation with the Russian Foreign Minister Sergey Lavrov on Wednesday morning, the US Secretary of State John Kerry “expressed grave concern” over the “attacks on hospitals, the water supply network, and other civilian infrastructure in Aleppo” by the Russian and Syrian forces, State Department spokesman John Kirby said.

“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,” Kirby added.

The Russian foreign minister pointed out that a number of anti-government groups described by Washington as “moderates” refused to follow the ceasefire arranged by Russia and the US on September 9, instead choosing to side with Jabhat Al-Nusra and attack the Syrian Army alongside this Al-Qaeda branch.

Al-Nusra commander says US supports jihadists indirectly, provides tanks & artillery via allies https://t.co/GMvtFcKrKS@Gayane_RT — RT (@RT_com) September 27, 2016

Washington is preparing to suspend the “bilateral engagement” on Syria with Moscow, including the establishment of the Joint Implementation Center, unless Russia immediately halts the attack on Aleppo and restores the ceasefire, Kerry told Lavrov.

The US promised long ago to separate the rebels from terrorists and it needs to live up to that obligation, Lavrov told Kerry, bringing up the recent interview of an Al-Nusra commander about how the group is receiving outside support – including American weapons – as well as the statement of a Syrian opposition leader Riyad Hijab that he did not consider Al-Nusra terrorists.

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In addition to Lavrov’s comments, Russian Foreign Ministry spokesperson Maria Zakharova on Wednesday said that a comprehensive solution for the Syrian crisis may be reached only if the US takes more constructive steps in the process.

“It all has now come to a critical point – either they fulfill their obligations, or the process can really slow down very seriously,” Zakharova said in an interview with Russian tabloid “Komsomolskaya Pravda.” “There is apparently a [side] in the United States, for which any real cooperation does not seem profitable, as does any evidence that it exists,” she said.

Kerry’s belief that Russia was responsible for the ceasefire’s collapse is “absolutely mistaken” author and journalist Abdel Bari Atwan told RT.

It was Washington that did not follow through on to the deal, failing to separate what it called moderate opposition from terrorist groups, he said.

“I’m really scared about this ultimatum, because it seems like the US administration is going to escalate the [situation] in Aleppo,” Atwan told RT. “Things are really moving into a really dangerous zone here.”