South Korean actress Jun Ji-hyun's photo appears on dozens of billboards in a Shanghai subway station, and she holds up the product she's plugging – an Oppo, the country's top-selling smartphone. It looks like a typical celebrity endorsement, but there's a backstory.

Ads featuring Korean stars, once ubiquitous in China, have started to dwindle. And this particular ad might not be around for long.

Advertising, along with TV entertainment and the music industry, has been affected by China's reported clampdown on South Korean pop culture. Though there has been no official policy change, several advertising executives said TV stations will no longer run ads featuring Korean stars. Obviously, that has made hiring them much less appealing to brands. As one executive creative director in China put it, the era of Korean celebrity endorsers "is finished."

For years Korean actors and singers developed lucrative side careers advertising products in China, where Korean music and movies are popular. Brands from Coca-Cola to Dove chocolates used them in hope that Korea's brand of cool would rub off on them.

Ms. Jun herself once appeared in an ad for KFC, the biggest Western fast-food chain in China. She has wide appeal in China, where her series "My Love From the Star," about a human actress in love with an alien, was a megahit. But an agency executive who works with Oppo said the brand will probably pull ads with Korean celebrities any time now, given the current climate. Oppo didn't respond to emails seeking comment.

Unusual punishment

"Hallyu," the Korean wave of K-pop and TV dramas, has been the unlikely target of a diplomatic dispute. Seoul said in July that it would put a U.S. missile defense technology on its soil; the technology, called a Terminal High Altitude Area Defense system, or THAAD, would offer protection from the country's neighbor, North Korea. But Beijing has opposed the plan, concerned about an impact on its own national security.

There were reports starting in August that China planned to show its displeasure by cracking down on Korean cultural imports. Since then media have reported on China banning TV content with Korean stars, including editing them out of shows. Reports also say Korean singers have not been granted clearance to perform in China.

Beijing has denied any new policy: a Foreign Ministry spokesman said in November that he hadn't heard of any restrictions.

But South Korea is concerned, and one of its senior diplomats raised the issue with China late last month, according to Korea's Yonhap news agency.

Impact on advertising

In terms of advertising, the unspoken ban on Korean stars applies to TV only, several executives in the industry said. Korean entertainers are still allowed to appear in social media, digital advertising or in print, for example. But hiring Korean stars is expensive, and it's not cost-effective if you can't put them on TV too, an executive creative director said. The TV ban has discouraged their use in general.

Some reports said Chinese actress and social media star Angelababy would replace Ms. Jun in the Oppo campaign, but the agency executive who works with Oppo pointed out that Angelababy is already plugging another smartphone brand. Oppo will likely just swap in its existing local celebrity endorsers for Ms. Jun, he said.

In August, China Daily newspaper reported that another top smartphone brand, Vivo, had substituted Taiwanese star Eddie Peng in for Song Joong-ki, the star of "Descendants of the Sun," a Korean military romance with a massive following in China. An ad industry insider said he wasn't replaced but that his campaign simply ran its course. Vivo did not respond to questions sent in by Ad Age.

Mr. Song was the most searched-for person online in China in 2016, according to the Baidu search engine. iQiyi, an online video platform owned by Baidu, said his show had 4.4 billion views as of the end of November. While that number seems hard to believe (there are 1.37 billion people in China), "Descendants of the Sun" was undeniably huge. The show was so popular that China's Ministry of Public Security warned about the dangers of watching too many Korean dramas; it said they had sparked divorces and domestic violence and led some to seek out plastic surgery.

There may, in fact, be a bit more to China's crackdown on Korean pop culture than the dispute over the missile defense system.

Jerome Mazet, founder and managing director of Mandaray, which connects brand sponsors to celebrities and sports teams, believes the THAAD explanation is "just a diversion," and that China's clampdown on popular hallyu may be intended to give more room for its own local pop culture to thrive.

China sees building its own entertainment culture as an essential tool to building its influence globally, he said: "The real plan is to become the dominant pop culture in Asia and then in the world."