The move by major marketers like AT&T, Verizon, Johnson & Johnson and GlaxoSmithKline to halt spending on YouTube due to ads running next to unsafe content is pushing media agencies to pressure Google to create a safer environment for advertisers.

The biggest concern is over Google's self-policing process and at least several agencies are reviewing how they are working with Google moving forward.

"Brand safety is a critical factor across all channels that we manage for our clients. Our immediate priority has been to work with Google to address our concerns and hold the organization accountable for any questionable activity," a Horizon Media spokeswoman said.

Horizon is in the process of working with Google and its YouTube third-party data partners to conduct a brand safety audit against its Google Direct YouTube buys across all clients.

"We have communicated to Google that their self-policing processes are not adequate and we expect Google to announce new safety precautions in the coming days," the spokeswoman said. "Until those new safety precautions are evaluated, we have recommended to our clients that we reevaluate their campaign activity."

Publicis Media told Ad Age in a statement that its "reviewing how we work with [Google] moving forward."

The media network, which is comprised of Mediavest Spark, Zenith, Blue449, Starcom and Performics, added in the statement that Publicis Media "is committed to being at the forefront of rigorous brand safety, viewability and verification standards and protocols."

"We hold all publishers, including Google and YouTube, accountable to ensure that the highest standards of advertising are consistently met. Clearly, Google has fallen short of these standards," the statement continued.

WPP's GroupM, which houses MEC, Maxus, Mindshare, MediaCom, Essence and more, said in a statement that the network "welcomes the commitments" around expanded safeguards for advertisers that Google Chief Business Officer Philipp Schindler wrote about in his blog on Tuesday.

The statement highlights the fact that Google has promised "improved brand safety systems to detect, identify and eliminate unacceptable content." GroupM added that it looks forward "to the details about the tools and a sense of what improvement benchmarks we should expect," and hopes for "quick delivery of the tools that can be deployed, as many thousands of campaigns are involved."

GroupM, the world's largest ad buyer, has been working with GlaxoSmithKline on media for several years. The group declined to comment on its client's decision to pull ad spending from Google.

MediaCom USA Chief Digital and Analytics Officer Steve Carbone told Ad Age that the agency, which takes brand safety seriously, has not had any clients pull advertising from YouTube or Google at this time. "As per GroupM's statement, advertising on platforms where content is user-generated and not curated, like YouTube, poses inherent brand safety risks," said Mr. Carbone. "We vigorously pursue every brand safety precaution and technology available to mitigate these risks, and we encourage our clients to make use of these tools."

Mr. Carbone said the agency has encouraged the development of solutions with Google, Facebook, Snapchat and other partners, but "a 100% foolproof system may not be possible" and "it's important that brands know this and proceed with caution, and with every protection possible."

Omnicom Media Group, which includes OMD, PHD and Hearts & Science, said in a statement: "Google's proposed actions hold the promise of enhanced brand safety and an improvement in advertiser confidence. Having said that, the proof of these measures remains to be seen. Therefore, together with each of our clients, we are carefully and continually assessing the situation."

The group declined to comment on the fact that Hearts & Science client AT&T halted its advertising with Google for anything but keyword search on Wednesday.

IPG Mediabrands' Magna Global committed to spend $250 million on Google Preferred, which offers premium inventory on YouTube, between October 2016 through December 2017, shifting money out of TV. The situation does not impact this committment, said David Cohen, president-North America, Magna Global, noting that 99.9% of Google Preferred inventory is safe. And in instances where brand messaging is showing up, Mr. Cohen said the number of impressions being impacted are very low, in many cases under 100.

"It is worth saying this whole topic on brand safety in digital advertising has been going on for a long time," Mr. Cohen told Ad Age, adding that he has received screenshots of ads running next to questionable content on a regular basis throughout the past 20 years.

He added that the agency is pushing Google for a third-party company to help monitor and prevent ads from being served in questionable content, but has yet to come to a resolution on the matter.

Last week, Johnny Hornby, founder of WPP-backed The&Partnership, called for advertisers to withdraw from YouTube and unsafe brand environments at the Guardian Media Summit unless platforms, like Google, allowed independent third-party ad verification.

Mr. Hornby said none of his clients have had to pull ad spending with YouTube because he has actively advised them not to spend there since the platform doesn't allow ad verification software.

According to a study commissioned by The&Partnership and m/SIX last year and conducted by technology company Adloox, ad fraud may have cost advertisers $12.48 billion globally in 2016, accounting for nearly 20% of the $66 billion spent on digital advertising.

The&Partnership has not recommended for advertisers to pull from Google Search because Mr. Hornby said Google Display and Google Exchange allows third-party verification software. However, he added that Google "should more than allow it, but recommend it or insist on it."

Mr. Cohen said Google's blog post was too vague for clients to feel like they're being heard, and while the company offers some fixes, it didn't provide a full solution.

"Right now we are taking a calm, thoughtful, measured approach and trying to do what's smart," he said. "We do not see wholesale clients flocking away from the platform. But if we don't see Google do more I can't say that won't be the case."

Hearst-owned iCrossing is advising clients away from pulling spending with YouTube at the moment and rather focus on "safeguarding their brand" with accredited verification partners to monitor where ads are running. Mike Racic, president-media services iCrossing, said in a statement that "moves to alienate YouTube ads are short sighted," adding that "the targeted reach opportunity lost by pulling ads from YouTube -- or programmatic platforms -- outweighs the few isolated incidents that are often sparked by media coverage."

For Google's display networks, however, the agency is "recommending pulling back from any non-premium, open ended buying."

"We might never arrive at a point where the digital environment is 100% clean of these issues given the pace of technology advancement in media and marketing," said Mr. Racic. "But what can be achieved immediately are advertisers and their partners raising their expectations and standards as we work through these types of challenges."

Representatives from Havas, which helped jumpstart this controversy last week when it pulled its U.K. clients' ads from Google and YouTube, did not respond to inquiry for comment.