The United States threatened Wednesday to suspend its engagement with Russia over the conflict in Syria following escalating attacks on rebel-held parts of Aleppo city, including strikes on two hospitals.

UN chief Ban Ki-moon denounced the attacks -- which saw the two largest hospitals in Aleppo's opposition-controlled east hit with air strikes and artillery fire -- as "war crimes".

President Bashar al-Assad's forces and his ally Moscow have carried out a barrage of air strikes on eastern Aleppo since Syria's regime announced a bid last week to retake all of the divided city.

Russia's defence ministry said Moscow was prepared to relaunch talks with the United States on the crisis, but a Syrian opposition official said a political solution to the conflict was "no longer a viable option".

Dozens of civilians have been killed, residential buildings have been reduced to rubble and residents of east Aleppo -- already suffering under a government siege -- are facing severe shortages.

The latest bombardment has been some of the worst in Syria's five-year civil war, and comes after the failure of a short-lived ceasefire brokered by Russia and the United States this month.

Moscow and Washington have traded blame over the truce's collapse, with the US harshly criticising Russia's participation in the Aleppo offensive.

On Wednesday, US Secretary of State John Kerry warned Russian counterpart Sergei Lavrov that Washington will end talks on the conflict unless Moscow halts the Aleppo assault.

Kerry said the burden was on Russia to stop the assault and ensure humanitarian aid access, his spokesman John Kirby said.

"The United States is making preparations to suspend US-Russia bilateral engagement on Syria... unless Russia takes immediate steps to end the assault on Aleppo and restore the cessation of hostilities," Kirby said.

- Solution 'no longer viable' -

Russia's defence ministry said later Moscow was ready to relaunch talks with Washington.

"On orders of the Russian president, we are ready to continue joint work with our American partners on the Syrian issue," and to send experts to Geneva to "relaunch consultations," it said.

But a member of the key National Coalition opposition movement said the escalating violence, particularly against rebel-held Aleppo, meant a political solution was "no longer viable".

Coalition vice-president Muwaffaq Nyrabia said rebels were "considering all options to defend the Syrian people against the Russian aggression on Syria".

Wednesday's attacks saw the M10 and M2 hospitals hit before dawn, forcing both to shut temporarily, said Adham Sahloul of the Syrian American Medical Society, which supports both hospitals.

Doctors Without Borders -- a charity known by its French acronym MSF -- said the two attacks killed at least two patients, wounded two medics and left just two of east Aleppo's eight hospitals with surgical facilities.

It was unclear who had carried out the bombings, which UN chief Ban denounced before the Security Council.

"Those using ever more destructive weapons know exactly what they are doing. They know they are committing war crimes," he said.

Inside one of the hospitals, patients and medical staff cowered in fear.

"I am in the M2 hospital now. I was inside when the entrance to the emergency room was hit. Three of my colleagues were hurt," Aref al-Aref, a medical assistant, said.

"Everyone is terrified and scared today. We are afraid that we will be today's victims."

Sahloul warned the attacks could have devastating consequences.

"With these two hospitals gone, if today there is another offensive like Saturday or Sunday, this is signing the death warrant for hundreds of people," he said.

- 'Opportunity to escape' -

The UN children's agency UNICEF said at least 96 children have been killed and 223 wounded since Friday in the rebel-held sections of Aleppo where the health system was crumbling with only 30 doctors left.

More than 170 people have been killed in east Aleppo since Syria's army announced its operation to retake the city, said the Syrian Observatory for Human Rights.

On Wednesday, at least six civilians died in artillery fire near a bakery in the opposition-controlled Maadi district, the monitor said.

A hospital in the government-held west reported two people had been killed and 10 wounded in rebel fire on the Aziziyeh district.

Clashes also continued inside Aleppo's Old City, the Observatory said, after pro-government troops seized control of the Farafira district northwest of Aleppo's historic citadel on Tuesday.

Once Syria's commercial hub, Aleppo has been ravaged by fighting and divided since mid-2012.

An estimated 250,000 people still live in the east, which has been under devastating siege by government forces since early September.

The head of the White Helmets volunteer rescue force, which operates in opposition-held territory in northern Syria, said that under current conditions civilian facilities in eastern Aleppo would no longer be able to function within a month.

"The civilians there would seize any opportunity to escape, to go wherever they could go," Raed Saleh told AFP.

"But nothing is available to provide safety and protection for those civilians."

kar-sah/ah/dv

KERRY GROUP