BEIJING—A new challenge looms for Apple Inc. in China after internet regulators warned three video-app companies to do a better job of screening out pornography—an eye-popping task, as is evident here in the offices of the popular Huajiao streaming service.

A team of workers, tasked to police output, watches as images of young women and other video performers flash across their computer screens every two seconds in grids that allow them to watch 60 shows at a time. The mission: to make sure the coquettish video stars don’t do anything to violate China’s ban on steamy content.

“It’s tough because a girl can be talking normally and then suddenly take off her clothes,” said a worker.

When the Cyberspace Administration of Beijing issued the warning Tuesday to Huajiao and two other app companies to improve censorship, it also said it planned to summon Apple executives to discuss stricter oversight of the company’s App Store. Under regulations issued last year, app stores in China share responsibility for ensuring content is legal.

Apple is the only foreign company running a major app store in China. Its App Store includes video streaming services among its thousands of apps, but Apple itself doesn’t stream videos. Apple has said it follows local law about what content is illegal and must be censored. Apple declined to comment. The Beijing Cyberspace Administration didn’t reply to requests for comment.