GENEVA — The top United Nations human rights official on Friday called the weekslong bombardment and siege of rebel-held parts 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 the besieged rebel-held eastern side of the city had 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 later adopted, 24 to 7, a resolution that called for an immediate end to the bombing of Aleppo. The resolution also asked the United Nations commission of inquiry monitoring human rights in Syria to investigate events in the city, identifying those responsible for war crimes and other violations. Sixteen council members abstained.