Facebook Inc. is once again in hot water for allowing objectionable videos on its website, this time drawing a rare rebuke from a United Nations agency.

The U.N. migration agency hammered the social network for allowing what it said was a “horrifying” video showing gangs in Libya threatening to harm a group of terrified migrants, many from Somalia and Ethiopia.

One migrant in the video is lying on his chest with a concrete block on his back, and that the video shows the faces of some migrants without their consent in a way that they can be identified.

The smugglers also are issuing threats to the migrants’ families over WhatsApp, a messaging app owned by Facebook, according to Leonard Doyle, spokesman for the International Organization for Migration.

Mr. Doyle said the video surfaced in June, after which his organization issued a press release condemning it. He said that should have prompted Facebook to take down the video.

“On what possible grounds can you show these poor, vulnerable people?” he said in an interview, adding that it was very rare that the U.N. agency would specifically criticize Facebook in this manner.

Facebook officials reached out to his agency on Friday to discuss the issue after the Times of London published a story on the video, but the company as of Sunday hadn’t removed the video.

Facebook said it allowed the video to stay up because the footage was shared by a journalist to shed light on human rights violations around the world.

The company said part of its role is to be a place where its more than 2 billion monthly users can raise awareness about important issues.

“This specific video was posted to condemn smuggling and raise aware (awareness) of the issue, so we would not consider it a violation of our policies,” a Facebook spokeswoman said in an email.

“We realize the video is disturbing so we have added a warning screen and the video’s distribution will be limited to those aged 18 and over.”

The episode marks the latest in a string of examples of Facebook’s video tools being used to promote and showcase violence and revives questions about Facebook’s handling of sensitive videos.

Earlier this year, Facebook was widely criticized by users and civil rights groups for allowing violent videos to be broadcast live on its platform, including one showing a man in Cleveland committing murder and one of a man in Thailand murdering his baby daughter.

Mr. Doyle said that Facebook has been slow to take down pages on its service run by smugglers trying to reach “vulnerable” people in Somalia and Ethiopia, trying to secure safe passage to Europe.

Mr. Doyle said Facebook should move faster and be more proactive in looking for instances where users are using Facebook to exploit vulnerable people. “Now that you’re reaching people, you actually have a responsibility to inform them that not everyone they meet on Facebook is their friend,” Mr. Doyle said.

“Offering services to take part in, support or promote people smuggling on Facebook is against our community standards,” the Facebook spokeswoman said in the email.

Content moderation experts say it is difficult for tech companies to uphold their standards because of the sheer amount of content posted online every day and the comparatively small number of content reviewers at the company. In May, Facebook said it would add 3,000 content monitors to review what is being posted online.

Mr. Doyle, echoing other critics, said Facebook has an “absolute responsibility” to invest the resources required to handle the wide range of sensitive issues found on Facebook.