GENEVA — The top United Nations human rights official on Friday called the weekslong bombardment and siege of Aleppo “crimes of historic proportions” that had turned the ancient Syrian city into a “slaughterhouse.”
As a “humanitarian pause” in attacks, declared unilaterally by Russia, entered a second day — and as residents of the besieged rebel-held eastern side of the city continued to enjoy a respite from the Syrian and Russian airstrikes that have devastated the area — the United Nations official, Zeid Ra’ad al-Hussein, called for a war crimes investigation.
The comments by Mr. al-Hussein, the high commissioner for human rights, came in a videotaped statement at the opening of a special session of the United Nations Human Rights Council in Geneva.
The 47-member council looked set to adopt a resolution calling for a United Nations commission investigating human rights developments in Syria to identify those responsible for war crimes and other violations in Aleppo.
Opposition groups have shelled government-controlled civilian areas of western Aleppo, but an overwhelming majority of civilian casualties have come from Russian and Syrian airstrikes on the rebel-held eastern areas, Mr. al-Hussein said.
“No hypothetical advantage in global gamesmanship could possibly outweigh this pain and horror,” he said.
“Every party to the conflict should know they will be held accountable for the international crimes they commit,” he added, urging the Security Council to promptly refer the conflict to the International Criminal Court.
Britain and the United States, among other countries, had called the session to increase international pressure for a halt to hostilities in Syria.
Aleppo is a major prize in the five-year Syrian war and the crux of a dispute between Russia, which backs the Syrian government, and the United States, which supports some rebel groups. Russia has long demanded that the United States force the groups it supports to separate themselves from fighters for the Levant Conquest Front, the Qaeda-associated group once called the Nusra Front.
The United Nations special envoy for Syria, Staffan de Mistura, has offered to personally escort the Nusra fighters, who number several hundred among about 8,000 rebels, he says, to the insurgent-held province of Idlib if the government allows the other fighters and civilians to stay in eastern Aleppo and govern themselves. There is no sign that either side would agree to that proposal.
“Russia, you are making the situation worse, not solving it,” Tobias Ellwood, Britain’s junior foreign minister for the Middle East and Africa, told the council. He condemned as “shameful” its veto of a Security Council resolution this month demanding an immediate end to the bombing of Aleppo.
Alexey Borodavkin, the Russian ambassador to the United Nations in Geneva, shot back that the resolution had failed to take into account the latest pause in hostilities. He called on the body to support the Syrian government’s fight against terrorists, instead of letting them regroup. Rebel groups, Mr. Borodavkin said, were holding residents hostage.
Few civilians in eastern Aleppo took advantage of the pause to leave the city. “From what I hear, people do not want to leave their places, they do not want to become refugees, they want to stay in their place,” Mr. de Mistura told reporters in Geneva as the Russian pause in attacks began.
Residents took advantage of the pause in airstrikes to demonstrate on Thursday night inside rebel-held areas, carrying signs with slogans like “No to Systematic Displacement.”
But temporary pauses in the fighting, which Russia has committed to observe for four days, were not sufficient to provide adequate relief for more than a quarter of a million people besieged in eastern Aleppo for two months, Paulo Sérgio Pinheiro, chairman of the United Nations panel monitoring human rights in Syria, told the council. He called for a sustained, unconditional cease-fire.
Humanitarian agencies stand ready to carry out emergency medical evacuations from rebel-held parts of the city, but they need more security guarantees from the combatants before they can begin, international aid officials said on Friday.
“So far, we could not proceed due to lack of security guarantees that can ensure the safety of the patients,” said Ingy Sedky, a spokeswoman for the International Committee of the Red Cross who was in the government-held part of western Aleppo to prepare for possible operations with its local partner, the Syrian Arab Red Crescent.
A coalition of rebel groups issued a statement declaring that it supported humanitarian operations if they included aid delivery. Separately, rebel leaders told reporters that they would not accept emergency medical evacuations if food and medicine were not also delivered to people trapped inside.
United Nations officials have said that forcing people to leave their homes in order to receive aid is against international law. But in practice, the government has taken back a number of areas with starve-or-surrender tactics, bombarding and starving people until they agree to leave.
In any case, without guarantees and a halt to shelling and shooting by all sides, aid agencies cannot begin to provide the medical evacuations and humanitarian aid deliveries to eastern Aleppo that United Nations officials said they would aim to carry out after Russia agreed to extend its cease-fire for several days.
Previous negotiations to evacuate civilians or fighters from besieged areas and to deliver aid, including in the city of Homs in 2014, faced difficulty in winning consensus among fighters on each side.
In Homs, there were disagreements among rebel groups, with some taking a harder line and pressuring other fighters and civilians to hold out longer. And even when the government of President Bashar al-Assad approved aid delivery to Homs, pro-government militias shelled a convoy.
Some residents have also said they are afraid to go to government-held areas because they may be wanted by security forces for demonstrating against the government or for supporting the rebellion; they want an option to go to rebel-held areas, or United Nations supervision of any evacuation.
There appeared to be confusion about how long the pause in hostilities would last. United Nations officials said that Russia had agreed to a four-day suspension, 11 hours a day, that would end on Monday, and that they had asked for an extension of a fifth day. But a text message sent by the Syrian and Russian governments to cellphones in eastern Aleppo said the pause was effective for eight hours a day, and that residents had until 4 p.m. on Saturday to leave.
“Use this opportunity,” the message said.