UNITED NATIONS — Russia said on Sunday morning that it would block an effort to deploy United Nations observers in Syria to witness the evacuation of thousands of men, women and children from eastern Aleppo, or what the United Nations chief, Ban Ki-moon, has described as “a synonym for hell.”

The showdown inside the Security Council came after senior United Nations officials pleaded for access to the evacuation process, in an attempt to verify a steady stream of reports of atrocities and dead bodies on the streets.

Last week, the United Nations top envoy to Syria, Staffan de Mistura, said that he had repeatedly asked the Syrian government for permission to allow United Nations staff members to witness the evacuation of civilians and fighters — but that no permission had been received.

On Sunday, his humanitarian adviser, Jan Egeland, repeated that call. “It is important that there are third parties present to witness and assist,” Mr. Egeland said in an email.