Oren Dorell, and Jane Onyanga-Omara

USA TODAY

Secretary of State John Kerry on Wednesday warned that the United States would stop talking to Moscow about ending Syria's civil war unless it ends the onslaught in the city of Aleppo by Russian and Syrian government forces.

Kerry's warning came as Pope Francis assailed the assault on civilians in what was Syria's largest city, saying the perpetrators will be held “accountable to God” for their actions.

Over the past week in Aleppo, Russian and Syrian war planes have unleashed the worst aerial bombardment in the 5-year-old war. More than 200 people have been killed, according to human rights groups.

Kerry, in a phone call Wednesday with Russian Foreign Minister Sergey Lavrov, repeated his demand that Russia press Syria's government to stop attacking opposition forces and civilians. He also warned that the U.S. "is making preparations to suspend U.S.-Russia bilateral engagement on Syria" unless that happens, State Department spokesman John Kirby said in a statement.

That means the U.S. will not participate with Russia in intelligence sharing and targeting of Islamic State and other terrorists, as planned under a cease-fire negotiated between the U.S. and Russia, and it could mean an end to negotiations in Geneva about finding a peaceful end to the conflict, Kirby told reporters at the State Department.

Kerry has been calling for an end to the attacks since they began Sept. 19, causing a week-old cease-fire negotiated between the U.S. and Russia to collapse.

The consequences for Russia's continued attacks on civilians in Aleppo could extend beyond the suspension of diplomacy with the United States, Kirby said.

"They’ll end up being — more deeply involved in this, and the war won’t stop," Kirby said. "Opposition groups are certainly not going to pull back, extremist groups are likely going to expand and take advantage of the chaos…. And more Russian resources will be expended, more Russian lives will be lost, more Russian aircraft will be shot down."

Kerry "made clear" the United States holds Russia responsible for the carnage, especially for using incendiary and bunker-buster bombs in an urban environment, which Kirby called "a drastic escalation that puts civilians at great risk." The deteriorating situation has been exacerbated by continued‎ Russian and Syrian government attacks on hospitals, the water supply and other civilian infrastructure in Aleppo, he said.

Lavrov countered that a number of anti-government groups described by Washington as “moderates” refused to follow the cease-fire and chose to side with al-Qaeda's Syrian affiliate, the Nusra Front, according to Russian state-owned broadcaster RT.

The pope, speaking in St. Peter’s Square on Wednesday, said: “In expressing my deep sorrow and lively concern for what is happening in that already battered city — where children, the elderly, the sick, young and old, all are dying — I renew my appeal to everyone to commit themselves with all their strength to the protection of civilians as an imperative and urgent obligation.”

More than 250,000 civilians are thought to be trapped inside rebel-held parts of Aleppo, which had a pre-war population of more than 2 million. Syrian government forces carried out the biggest ground assault yet in their new offensive Tuesday, Reuters reported.

Civilian deaths mount as Aleppo siege by Syrian forces advances

Airstrikes hit a hospital in a rebel-held area early Wednesday, according to Al Jazeera.