Russian or Syrian warplanes knocked two hospitals out of service in the besieged rebel sector of Aleppo on Wednesday and ground forces intensified an assault in a battle which the United Nations said had made the city worse than a slaughterhouse.

Two patients died in one of the hospitals and other shelling killed six residents queuing for bread under a siege that has trapped 250,000 people with food running out.

The week-old assault, which could herald a turning point in the war, has already killed hundreds of people, with bunker-busting bombs bringing down buildings on residents huddled inside. Only about 30 doctors are believed to be left inside the besieged zone, coping with hundreds of wounded a day.

"The warplane flew over us and directly started dropping its missiles ... at around 4 a.m.," Mohammad Abu Rajab, a radiologist at the M10 hospital, the largest trauma hospital in the city's rebel-held sector, told Reuters.

"Rubble fell in on the patients in the intensive care unit."

M10 hospital workers said oxygen and power generators were destroyed and patients were transferred to another hospital.

Photographs sent to Reuters by a hospital worker at the facility showed damaged storage tanks, a rubble strewn area, and the collapsed roof of what he said was a power facility.

There were no initial reports of casualties there, but medical charity Médecins Sans Frontières (MSF) said two patients had been killed at the other hospital, in shelling which took it out of service as well, leaving east Aleppo with only seven doctors in a position to undertake surgery.

“And this comes at a time when east Aleppo has been under siege since July and is suffering the bloodiest indiscriminate bombing since the beginning of the war," MSF's Syria head Carlos Francisco said.

The government of President Bashar al-Assad, backed by Russian air power, Iranian ground forces and Shi'ite militia fighters from Iran, Iraq and Lebanon, has launched a massive assault to crush the rebels' last major urban stronghold.

Syria's largest city before the war, Aleppo has been divided for years between government and rebel zones, and would be the biggest strategic prize of the war for Assad and his allies.

Taking full control of the city would restore near full government rule over the most important cities of western Syria, where nearly all of the population lived before the start of a conflict that has since made half of Syrians homeless, caused a refugee crisis and contributed to the rise of Daesh.

UNPRECEDENTED BOMBING

The offensive began with unprecedentedly fierce bombing last week, followed by a ground campaign this week, burying a ceasefire that had been the culmination of months of diplomacy between Washington and Moscow.

Washington says Moscow and Damascus are guilty of war crimes for targeting civilians, hospitals, rescue workers and aid deliveries, to break the will of residents and force them to surrender. Syria and Russia say they target only militants.

Asked by a reporter at the United Nations whether Syria had bombed the two hospitals hit on Wednesday, the Syrian ambassador to the world body, Bashar Ja'afari, appeared to laugh.

The Syrian army said a Nusra Front position had been destroyed in Aleppo's old quarter, and other militant-held areas targeted in "concentrated air strikes" near the city.

UN Secretary-General Ban Ki-moon said those using "ever more destructive weapons" were committing war crimes. Describing the situation in Aleppo, he said: "This is worse. Even a slaughterhouse is more humane."

France's Foreign Minister Jean-Marc Ayrault said he was working to put forward a United Nations Security Council resolution to impose a ceasefire in Aleppo, and that any country that opposed it would be deemed complicit in war crimes.

"This resolution will leave everyone facing their responsibilities: those who don't vote it, risk being held responsible for complicity in war crimes," he said.

Another hospital, M2, was damaged by bombardment in the al-Maadi district, where at least six people were killed while queuing for bread at a nearby bakery, according to the Syrian Observatory for Human Rights monitoring body and residents.

Food supplies are scarce in the besieged area, and those trapped inside often line up before dawn for food.

The collapse of the peace process leaves US policy on Syria in tatters and is a personal blow to Secretary of State John Kerry, who led talks with Moscow despite skepticism from other top officials in President Barack Obama's administration.

Kerry spoke to Lavrov on Wednesday and Russia said later that it was ready to continue diplomacy on Syria and would soon send experts to Geneva for talks with US counterparts on normalizing the situation in Aleppo and elsewhere.

The US State Department said non-diplomatic options to halt the violence had been discussed within the administration, but declined to say what the options might be.