Like many Super Bowl ads, Hyundai's salute to U.S. troops, which appeared just after the game, got considerable exposure later in the week on YouTube, including where the brand never intended – as pre-roll to a video supporting Hezbollah.

The Iranian-backed group, designated a terrorist organization by the U.S., is among other things believed responsible for the 1983 truck bombing of a Marine barracks in Beirut that killed 241 U.S. service personnel on a peacekeeping mission.

Hyundai was among several major advertisers last week that had their ads appear alongside videos that appeared to support terrorism, the latest cases of a recurring problem that prompted a call from the World Federation of Advertisers for Google and others to be more vigilant, and a sign that efforts by marketers to stay clear of unsavory digital content aren't always working.

Hyundai finds the YouTube placement "highly concerning," said Jim Trainor, director-communications for Hyundai Motor America, in an e-mail statement on Friday. "We've been working with Google to find out how it happened this time and to ensure that it doesn't happen again. We have strict guidelines when working with Google. Our 'blacklist' [never-to-appear-on list] overrides all other criteria to ensure that violent or extremist sites never serve our content."

Several other Super Bowl ads also appeared post-game either in pre-roll or right-rail placements alongside the Hezbollah video or a separate "Indian Nasheed" whose English subtitles encouraged violence against the Pakistani army for its actions against Islamic militants. Those included ads from AB-InBev's Budweiser and Bud Light, T-Mobile and, at least according to screenshots provided by third-party monitoring firm Gipec, Airbnb and Procter & Gamble Co.

The "Indian Nasheed" video was removed from YouTube after an Ad Age inquiry to Google, though the video bearing the Hezbollah logo remains.

"We have clear policies prohibiting terrorist recruitment and content intending to incite violence, and quickly remove videos violating these policies when flagged by our users," a Google spokesman said in an e-mail statement. "We also have stringent advertising guidelines, and work hard to prevent ads appearing against any video once we determine that the content is not appropriate for our advertising partners. When we become aware of ads that are being served against ineligible content we immediately take action to remove them."

A spokeswoman for T-Mobile said the placements of its ads on the videos in question weren't authorized by the company. A P&G spokeswoman said the company hadn't been able to verify yet whether its ads actually ran opposite the "Indian Nasheed" in a placement depicted in a screen capture by Gipec, a Michigan firm that monitors the internet for criminal activity and objectionable content. In other cases, Ad Age saw pre-roll ads from brands in question on the videos.

"We have not yet verified with Google if the ad ran as depicted, and that is an important first step because third-party information is not always accurate," a P&G spokeswoman said in an e-mail. "We are in contact with Google to investigate, and it appears that the video has been removed."

Spokespeople for Airbnb and AB InBev, marketer of Budweiser and Bud Light, didn't respond to email requests for comment on Friday.

The issue comes amid increased scrutiny about the integrity of the digital ad buying supply chain following P&G Chief Brand Officer Marc Pritchard's "call to action" speech to an Interactive Advertising Bureau gathering last month.

On Thursday, in response to a separate story by the Times of London about similar YouTube placements as well as programmatic ad buys on what appear to be sites promoting jihad or Nazism, the World Federation of Advertisers cited such placements as part of larger problems in the digital supply chain. The WFA specifically called out programmatic ad placement, which its research shows now makes up 16% of large multinational global advertisers' budgets, up from 10% two years ago.

"Both we and our members are increasingly concerned about the lack of transparency in this ecosystem," said WFA CEO Stephan Loerke in a statement. "This takes many forms; including brand misplacement (as highlighted in recent media reports) but also the endemic levels of ad fraud."

While brand owners may be taking steps to protect themselves, Mr. Loerke said, "it is ultimately incumbent on the platforms … to do more to restrict this sort of inappropriate content from appearing in the first place."

The Association of National Advertisers is also concerned. Preventing ad placements on objectionable content are part of the mission of the ANA-backed Trustworthy Accountability Group. Google was one of the initial recipients of TAG's "Certified Against Fraud" seal in December.

"Certain types of content, such as terrorist activity and IP infringement and pornography, they're all prohibited from having ads served against them" by TAG standards, said ANA CEO Bob Liodice in an interview.

Reports of ads placed alongside content supporting terrorist organizations are among examples, he said, of why "marketers have to take their industry back." But he said, "We need to recognize that we're still not winning enough," adding that "we need to keep advocating, pushing and investing behind protocols that will allow us to do that."

Gipec CEO Eric Feinberg said in an email that ad placements on YouTube, such as the Super Bowl ads, may help fund terrorist groups such as ISIS, a contention also made in the Times of London piece and a separate story Thursday from Fox News.

Ad Age did see pre-roll ads from two other major national advertisers running opposite a "nasheed" music video with an ISIS flag on Thursday, in addition to the other videos, though the exact content wasn't apparent. But in this, and the other cases, it's unclear who posted the videos or where the money shared by Google through its partner program actually goes.

Efforts to flag of objectionable content aren't always foolproof or easy either. In other cases, nasheed videos Mr. Feinberg cited, which feature vocal music on Muslim religious themes either a capella or with percussion instruments, did not have obvious exhortations to violence or association with designated terrorist organizations.