US healthcare conglomerate Johnson & Johnson has become the latest company to suspend all digital advertising on YouTube over concerns its ads may have appeared on channels that broadcast offensive videos.

J&J said on Thursday it wanted to ensure that its product advertising did not appear on channels that promote "offensive content", a day after AT&T, Verizon, and several other major advertisers suspended their digital ads after discovering their brands had been appearing alongside videos promoting violence and hatred.

"We are deeply concerned that our ads may have appeared alongside YouTube content promoting terrorism and hate," AT&T said in a statement.

"Until Google can ensure this won't happen again, we are removing our ads from Google's non-search platforms."

Google, YouTube's parent company, has come under intense scrutiny for ads appearing alongside videos carrying homophobic or anti-Semitic messages.

Last week, an investigation by The Times in London newspaper revealed the ads of major brands were appearing in videos delving into contentious themes.

The company has vowed an overhaul of its practices and said it started an extensive review of its advertising policies.

Shares of Alphabet, Google's parent company, fell by 1.3 percent, or $11.21 per share following J&J's announcement.

YouTube has become one of the fastest growing parts of Google's ad system, which generated $79bn in revenue last year.

Google hasn't disclosed how much of that came from YouTube ads, but research firm eMarketer estimated the video site accounted for $5.6bn.

EMarketer projected YouTube's advertising will rise 26 percent this year to $7bn, but that prediction came before marketers began to suspend their spending.