Johnson & Johnson and JPMorgan Chase & Co have become the latest big US companies to suspend all digital advertising on Google's YouTube over concerns its ads might have appeared on channels that broadcast offensive videos.

Wireless carriers Verizon Communications Inc and AT&T Inc said on Wednesday they would suspend digital ads on YouTube, joining a list of well-known British brands such as retailer Marks and Spencer Group Plc that are deserting Alphabet Inc's Google.

Google has come under intense scrutiny for ads appearing alongside videos on YouTube carrying homophobic or anti-Semitic messages.

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

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Shares of Google parent Alphabet ended down 1.2 per cent on the New York Stock Exchange on Thursday.

Control over online ad placement has become a hot-button issue for advertisers, with social networks and news aggregators coming under fire during and after the US presidential election for spreading fake news reports.

Advertisers have also sought to avoid having their brands appear beside content they consider hate speech.

J&J said on Thursday it wanted to ensure that its product advertising did not appear on channels that promote "offensive content".

JPMorgan, the biggest US bank by assets and the biggest issuer of general purpose credit cards, suspended all ads from YouTube on Thursday, according to spokeswoman Trish Wexler.