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.