UNITED NATIONS — Russia and Syria faced a growing chorus of international condemnation over their actions in Aleppo, but U.N. officials conceded Thursday there was little they could do beyond shaming the two countries for what they are increasingly calling “war crimes.”

U.N. humanitarian chief Stephen O’Brien told the Security Council that with the Syrian army’s encirclement, eastern Aleppo has descended into the “merciless abyss of humanitarian catastrophe,” with 275,000 additional Syrians now being besieged.

“Besiegement it’s not a weapon of war, it is a flagrant, unjustifiable breech of the law — law which the besieging parties have signed up to,” O’Brien said. “Even if not today, one day there will be no hiding place for the individuals and institutions callously, cynically perpetrating these war crimes.”

He said the total number of besieged people in Syria had climbed to 861,200 up from 586,000 just weeks ago.

O’Brien said both the Syrian government, which has encircled eastern Aleppo, and the rebels mounting attacks from inside the city were to blame for the situation.

He also rapped the council for its inaction.

“The only remaining deterrent it seems is that there will be real accountability in the court of world opinion and disgust, goodness knows nothing else seems to be working to stop this deliberate and gratuitous carnage of lives lost,” O’Brien added.

Attempts to take action at the U.N. to end Syria’s civil war, now in its sixth year, have repeatedly failed because Russia, Syria’s close ally, is one of the Security Council’s five veto wielding members.

On Wednesday, U.N. chief Ban Ki-moon also implicitly accused Syria and Russia of committing war crimes.

Without naming countries, Ban said “those using ever more destructive weapons know exactly what they are doing — they know they are committing war crimes.” The Syrian government announced the offensive to retake rebel-held eastern Aleppo, and the only countries carrying out airstrikes are Syria and Russia.

Before heading into the Security Council meeting, U.S. Ambassador Samantha Power described the situation as “grotesque.”

“You saw yesterday how the Syrian Ambassador feels about the death, the slaughter, of the Syrian people in Aleppo – he laughed,” Power said. “That’s the coldness and the brutality that we’re talking about, and Russia stands right alongside those individuals and those bombers and, again, is not backing the regime – is fighting alongside the regime, bombing alongside the regime, exceeding in brutality what we have seen from the regime in the life of this war.”

Syria’s U.N. Ambassador Bashar Ja’afari denied his government was behind the bombings of civilians and aid convoys and blamed “terrorists.”

Power took issue with Syria’s oft-repeated claim that terrorists were behind the carnage and described Syria’s actions as a gift to the Islamic State group and Al-Nusrah.

“Rescue workers are not terrorists. Hospital workers are not terrorists. What they are doing is sowing not only the doom of this country and these people and this proud civilization of Syria, but it is going to generate more refugee flow, more radicalization,” Power said.