The U.N. humanitarian chief accused Russia and Syria on Wednesday of using bombing and starvation tactics in eastern Aleppo to push people to surrender or to death, triggering an unusual verbal attack on a U.N. official from the Russian ambassador.

The verbal fireworks exploded after Undersecretary-General Stephen O'Brien briefed the U.N. Security Council on what he called "the apocalyptic horror" in rebel-held eastern Aleppo, where 400 people have been killed and nearly 2,000 injured in less than a month, many of them children.

He blamed Syria for besieging the city and at the same time carrying out a bombing campaign with its Russian allies in a deliberate campaign to "make life intolerable, make death likely."

Russia's U.N. Ambassador Vitaly Churkin said it was "outrageous" that O'Brien spoke as if the bombings in eastern Aleppo are going on now when they have stopped for seven days, and for creating "the impression that chemical weapons have been used" in the rebel-held part of the city.

He demanded the evidence and accused O'Brien of failing to mention that government-held western Aleppo "has been hostage to the terrorists of al-Nusra," the al-Qaida-linked extremist group.

Churkin told O'Brien that these and other omissions "make your statement unfair and dishonest."

The council was supposed to go into closed consultations after O'Brien's briefing. Instead Churkin, the current council president, allowed members to speak in the open session, which put a spotlight on the deep divisions that have made the U.N.'s most powerful body impotent in taking action to end the more than five-year Syrian conflict.

Referring directly to Russia without naming it, O'Brien called on "all council members who have operational military assets in Syria to take concrete steps to halt the aerial bombardment of civilian areas."

He also read from leaflets dropped on eastern Aleppo by Syrian and Russian aircraft which he said make their intention "chillingly clear."

"This is your last hope ... Save yourselves," the leaflets read. "If you do not leave these areas urgently you will be annihilated. ... They left you alone to face your doom and nobody will give you any help."

O'Brien demanded that all parties end "these medieval tactics" and grant humanitarian access to Aleppo and the hundreds of thousands of people trapped in 17 other besieged areas around the country.

The U.S., Britain and France strongly defended O'Brien and joined the attack on Russia.

"Does Russia believe that all the children who are being killed in eastern Aleppo are themselves al-Qaida members?" U.S. Ambassador Samantha Power asked.

And she told Churkin: "You don't get congratulations and get credit for not committing war crimes for a day, or a week. That's not how the international system is structured, and nor should it ever be."

At the end of the meeting, O'Brien was given the floor to respond.

"As an international civil servant I do not retract, qualify or disclaim any fact or part of my earlier statement," he said.

Addressing the Russian ambassador, he added, "I am prompted to state the age-old truth: Don't shoot the messenger. Stand up for the beleaguered people of Syria and ... stop the war now."