The growing advertiser boycott of YouTube has reached the US, with AT&T, Johnson & Johnson, Verizon and Enterprise all halting adverts on the site, as well as Google’s wider ad networks, over the video-sharing site’s inability to guarantee promotional messages won’t appear alongside extremist content.

Some of the new members of the boycott have also expanded the scope of the complaints, from a specific focus on YouTube to a broader criticism of Google’s ad platform. “We are deeply concerned that our ads may have appeared alongside YouTube content promoting terrorism and hate,” AT&T said. “Until Google can ensure this won’t happen again, we are removing our ads from Google’s non-search platforms.”

The companies have joined the boycott, which began in the UK with organisations including the BBC, the Guardian and the British Government, despite the efforts of Google to quell fears on Monday.

Google’s chief business officer has promised a three-pronged plan to improve its advertising network, and apologised to brands for placing their ads “on content that was not aligned with their values”. The executive, Philipp Schindler, promised to tighten up policies, offer better controls to advertisers, and use AI to review questionable content.

But the number of companies boycotting Google’s ad networks continues to grow, now standing at around 250 firms. As well as YouTube, it has expanded to Google’s other major advertising service, AdSense, which brands use to place adverts on third-party sites. The most important of Google’s revenue sources, AdWords, is unaffected for now. The service lets companies advertise on Google itself, targeting specific search queries.