Google has discovered Russian operatives spent tens of thousands of dollars on ads on its YouTube, Gmail and Google Search products in an effort to meddle in the 2016 US presidential election.
The ads do not appear to be from the same Kremlin-affiliated entity that bought ads on Facebook, but may indicate a broader Russian online disinformation effort, a person briefed on the company's probe told Reuters on Monday.
Microsoft Corp said separately on Monday that it was looking at whether Russians bought US election ads on its Bing search engine or other Microsoft-owned products and platforms.
A spokeswoman for the company declined to comment further.
The revelation about Google is likely to fuel further scrutiny of the role that Silicon Valley technology giants may have unwittingly played during last year's election.
US intelligence agencies have concluded that Moscow's goal was to help elect Donald Trump.
Google has uncovered less than $US100,000 ($A129,000) in ad spending potentially linked to Russian actors, the source said.
Both Twitter and Facebook recently detected and disclosed that suspected Russian operatives, working for a content farm known as the Internet Research Agency in St Petersburg, used their platforms to purchase ads and post content that was politically divisive in a bid to influence Americans before and after the November 2016 presidential election.
The Internet Research Agency employ hundreds of so-called "trolls" who post pro-Kremlin content, much of it fake or discredited, under the guise of phony social media accounts that posed as American or European residents, according to lawmakers and researchers.
Facebook announced last month it had unearthed $US100,000 ($A129,000) in spending by the Internet Research Agency and, under pressure from lawmakers, has pledged to be more transparent about how its ads are purchased and targeted.
Google's review had been more robust than ones undertaken so far by Facebook or Twitter, the source said.
Russia's ad purchases on Google were first reported by the Washington Post.
Google did not deny the story, and in a statement pointed to its existing ad policies that limit political ad targeting and prohibit targeting based on race or religion.
"We are taking a deeper look to investigate attempts to abuse our systems, working with researchers and other companies, and will provide assistance to ongoing inquiries," a Google spokeswoman said on Monday.