US Secretary of State John Kerry has threatened to cut off all contact with Moscow over Syria unless Russian and Syrian air strikes on the northern city of Aleppo end.

The State Department says Kerry issued the ultimatum in a telephone call to Russian Foreign Minister Sergey Lavrov on Wednesday.

Mr Kerry’s spokesperson John Kirby said in a statement that Kerry “expressed grave concern” over Russian backed Syrian government bombings on hospitals, medical centres, a water pumping station and other civilian infrastructure in rebel-held neighbourhoods.

On Wednesday morning, two strikes destroyed entire departments of east Aleppo’s two main hospitals, leaving patients in need of evacuation and wiping out facilities in the emergency departments and life support units.

The rebel area now only has 29 doctors working in three hospitals with functioning emergency response facilities. Photographs from earlier this week showed patients being treated on the floor in make-shift medical centres, the white tiles slick with blood.

Residents say the unprecedented attack is the worst they have ever witnessed. It has included the use of ground-penetrating bombs which blow up underground shelters, incendiary devices and cluster munitions which have killed approximately 500 people and left 2,000 in need of urgent medical care, the civil defence rescue service said.

The State Department said that it told Moscow it holds Russia responsible for the use of such weapons in a built up urban area in which 250,000 people are currently living under siege conditions, unable to flee.

Both Russia and Syria have said humanitarian corridors into government held Aleppo are open to civilians. However, people on the ground report that anyone approaching the siege barricades has come under sniper fire.

US Secretary of State John Kerry (left) and Russian Foreign Minister Sergei Lavrov holding talks (EPA)

Mr Kerry told his counterpart the US was preparing to “suspend US-Russia bilateral engagement on Syria,” including proposed joint air strikes against Isis and Jabhat Fatah al-Sham, or al-Nusra, “unless Russia takes immediate steps to end the assault on Aleppo” and helps to restore the failed ceasefire.

The US and Russian brokered Eid-al-Adha ceasefire broke down after suspected Russian or Syrian jets bombed an aid convoy near Aleppo on 19 September, killing 20 people. Both Moscow and Damascus denied carrying out the attack.

In turn, the two allies blamed the US for endangering the fragile ceasefire by accidentally bombing a Syrian army base near Isis territory a few days earlier which left 63 soldiers dead.

The Kremlin said last week there is “no prospect” of renewing talks, and US-backed rebel groups in Syria stated that they refuse to work with Russia as Moscow is a party to the “suffering of the Syrian people”.

Mr Kerry had previously defended his repeated efforts to reach out to Russia as a partner in staking a path to peace in the six-year-long Syrian conflict. Speaking to reporters on Monday, he said: “The cause of what is happening [in Aleppo] is [Syrian President Bashar] Assad and Russia wanting to pursue a military victory.”

Syria war: What is the importance of Aleppo?

“Today there is no ceasefire and we’re not talking to them right now. And what’s happening? The place is being utterly destroyed. That’s not delusional. That’s a fact.”

The air strikes, which began in a new offensive announced last Thursday, were followed up with ground troops which have clashed with rebels in several different parts of the city in the last 24 hours.

A Syrian military official told the AP that the offensive will continue daily on all fronts until “terrorists” in east Aleppo are “wiped out”.