U.S. Secretary of State John Kerry is defending his efforts to negotiate with Moscow over the war in Syria, despite the collapse of a ceasefire that has led to a massive Russian-backed assault on the besieged rebel-held sector of Aleppo.

Medical supplies were running out in eastern Aleppo, with victims pouring into barely functioning hospitals as Russia and its ally, Syrian President Bashar al-Assad, ignored Western pleas to stop the bombing of the last major urban area in opposition hands.

Moscow and Damascus launched their assault last week despite months of negotiations led by Kerry that resulted in a short-lived ceasefire earlier this month. The secretary of state's diplomatic overtures to Moscow had faced skepticism, including from other senior officials within the U.S. administration.

Kerry said Monday that his failed ceasefire was not the cause of the fighting, and the only way to stop the war was to talk. He lashed back at critics, including Republican senator John McCain, who described him last week as "intrepid but delusional" for putting too much faith in Russia.

"The cause of what is happening is Assad and Russia wanting to pursue a military victory," Kerry told reporters during a trip to Colombia. "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 Syrian government's offensive to recapture all of Aleppo, with Russian air support and Iranian help on the ground, has been accompanied by bombing that residents describe as unprecedented in its ferocity.

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One of the warning leaflets dropped by the Syrian army is seen in the rebel-held Tariq al-Bab neighbourhood of Aleppo. The leaflet reads: "This is your destiny!!!" and "Who is next" as it depicts pictures of killed rebel commanders and fighters. (Abdalrhman Ismail/Reuters)

Some 250,000 civilians remain trapped in what was once Syria's biggest city. Hundreds of people, including dozens of children, have been reported killed since Thursday night by an onslaught that includes bunker-busting bombs that bring down whole buildings.

In a tense confrontation at the United Nations over the weekend, the United States called Russia's bombing in support of Assad "barbarism," and said Russia was killing civilians, medical staff and aid workers.

Moscow and Damascus say they are bombing only militants, although video from Aleppo has repeatedly shown small children being dug out of the rubble of collapsed buildings.

Few doctors left in Aleppo

Inside the rebel-held sector of Aleppo, there are only about 30 doctors left, coping with scores of freshly wounded every day, according to a statement from the Syrian American Medical Society (SAMS) charity group.

"Aleppo city's hospitals are overwhelmed with wounded people ... Things are starting to run out," said Aref al-Aref, an intensive care medical worker.

"We are unable to bring anything in ... not equipment and not even medical staff. Some medical staff are in the countryside, unable to come in because of the siege," he said.

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A Syrian boy receives treatment at a makeshift hospital following airstrikes on Aleppo on Saturday. The Syrian American Medical Society (SAMS) charity group said this week that only 30 doctors remain inside the rebel-held region. (Karam Al-Masri/AFP/Getty Images)

Bebars Mishal, a civil defence worker, said overnight bombardment continued until 6 a.m. local time.

"It's the same situation. Especially at night, the bombardment intensifies, it becomes more violent, using all kinds of weapons, phosphorous and napalm and cluster bombs," Mishal told Reuters.

"Now, there's just the helicopter, and God only knows where it will bomb. God knows which building will collapse," he said. "Everybody is scared ... unable to go out. They don't know what to do, or where to go."

Russia and Assad appear to have abandoned diplomacy last week, betting instead on delivering a decisive military blow against the president's enemies on the battlefield. Capturing rebel districts of Aleppo would be the biggest victory of the war so far for Assad, crushing the revolt in its last major urban stronghold.

No respite

Indicating there would be no respite soon, the Syrian army issued a statement reiterating its call for civilians to steer clear of rebel positions and bases in eastern Aleppo.

The Syrian Observatory for Human Rights, a monitoring body, says at least 237 people, including at least 38 children, have been killed in Aleppo and nearby countryside since the army declared the end of the ceasefire a week ago.

Civil defence workers in opposition territory put the death toll at 400.

The rebel-held sector of Aleppo is completely encircled, making it impossible to receive supplies.

'Everybody is scared … unable to go out. They don't know what to do, or where to go.' - Civil defence worker Bebars Mishal

Residents say the airstrikes are using more powerful bombs than ever. A Syrian military source told Reuters Saturday that weapons were being used that could destroy rebel tunnels and bunkers, dug out during years of opposition control.

Rescue efforts during the bombing have been hampered because damage has made roads impassable and because civil defence centres and rescue equipment have themselves been struck.

Washington-Moscow diplomacy

Hundreds of thousands of people have been killed during the 5½-year-long civil war between Assad's government and insurgents, and 11 million driven from their homes.

Much of the east of the country is now in the hands of Islamic State (ISIS) fighters — the enemies of all other sides.

Since Russia joined the war a year ago to support Assad's government, the administration of U.S. President Barack Obama has been engaged in intensive diplomacy with Moscow, trying to end the war between the government and most insurgent groups and turn the focus toward the common fight against ISIS.

The Kremlin said Monday tough Western condemnation might hinder any resolution to the crisis. Moscow saw "absolutely no prospect" for holding a summit on Syria, Kremlin spokesman Dmitry Peskov said.

Moscow blames Washington for the failure of the ceasefire, arguing that the United States failed to prevent rebels from using the truce to regroup. 

Meanwhile, the United Nations World Food Program announced Monday it had delivered food assistance to four besieged Syrian towns over the weekend, marking the first time supplies had reached the communities since April.

"This convoy has brought relief for 60,000 people who are in dire need of food and medical supplies, and have been cut off from humanitarian access for five months," said WFP director Jakob Kern.

Forty-five trucks carrying rations such as rice, flour, sugar, salt and beans reached the towns of Madaya, Zabadani, Foaa and Kerfraya on Sunday, according to a WFP statement, adding that the rations should be sufficient for a month.

With files from CBC News