US Secretary of State John Kerry has threatened to cut all diplomatic ties with Russia, in a bleak deterioration in US-Russia relations this week.

The US State Department confirmed that Kerry issued an ultimatum to his Russian counterpart, Sergey Lavrov, warning him that unless Russian and Syrian bombardment of Aleppo ends, relations between Moscow and Washington D.C. would revert to Cold War levels.

Metro.co.uk reports:

Kerry’s spokesman, John Kirby, says Kerry expressed grave concern over Russian and Syrian government attacks on hospitals, water supplies and other civilian infrastructure in Aleppo.

He says Kerry told Lavrov that the U.S. holds Russia responsible for the use of incendiary and bunker-buster bombs in an urban area.

Kerry told Lavrov the U.S. was preparing to ‘suspend U.S.-Russia bilateral engagement on Syria,’ including on a proposed counterterrorism partnership, ‘unless Russia takes immediate steps to end the assault on Aleppo’ and restore a cease-fire.

Russian or Syrian warplanes knocked a major Aleppo hospital out of service today, hospital workers said, and ground forces intensified an assault on the city’s besieged rebel sector, in a battle that has become a potentially decisive turning point in the civil war.

Shelling damaged at least another hospital and a bakery, killing six residents queuing up for bread under a siege that has trapped 250,000 people with food running out.

The World Health Organization said it had reports that both hospitals were now out of service.

UN Secretary-General Ban Ki-moon condemned the attacks describing them as ‘war crimes.’

‘Let us be clear. Those using ever more destructive weapons know exactly what they are doing. They know they are committing war crimes,’ Ban told the Security Council.

‘Imagine the destruction. People with limbs blown off. Children in terrible pain with no relief,’ he said. ‘Imagine a slaughterhouse. This is worse.’

The two biggest hospitals in rebel-controlled parts of Aleppo have been bombed in what non-government organizations residents say are deliberate attacks by the Syrian regime and its Russian allies to eliminate these structures.

In May, the UN Security Council adopted a resolution on the protection of health workers and facilities during armed conflicts, but there has been no letup in these kinds of attacks in Syria and Yemen.

‘International law is clear: medical workers, facilities and transport must be protected. The wounded and sick – civilians and fighters alike – must be spared,’ Ban said.

‘Deliberate attacks on hospitals are war crimes. Denying people access to essential health care violates international humanitarian law.’

Ban cited statistics gathered by Physicians for Human Rights that show that 95 percent of the medical personnel who were in Aleppo before the war ‘have fled, been detained, or killed.’

‘There must be action. There must be accountability,’ he said.