IBM has suspended advertising on X, formerly known as Twitter, after a report said its ads were placed next to posts praising Adolf Hitler and Nazism.
The company said it was "completely unacceptable" that its content appeared in such threads on the platform.
X said it does not intentionally place brands "next to this kind of content".
It comes as X owner Elon Musk was criticised after calling an antisemitic conspiracy theory "actual truth" when replying to a post on the platform.
The left-leaning watchdog Media Matters for America said it found ads bought by IBM and other companies next to posts including Hitler quotes, praise of Nazis and Holocaust denial.
One pro-Nazi post that was shown next to an IBM ad was seen about 8,000 times, X said in a statement to the OceanNewsUK.
The other firms listed include Apple, Oracle, television network Bravo and telecoms company Xfinity. The OceanNewsUK has approached the companies for comment.
"IBM has zero tolerance for hate speech and discrimination and we have immediately suspended all advertising on X while we investigate this entirely unacceptable situation," the company said in a statement.
X told the OceanNewsUK that ads are not deliberately placed next to extremist content, that the Nazi-promoting accounts will not earn money from advertising and that specific posts will be labelled "sensitive media".
"X's point of view has always been very clear that discrimination by everyone should STOP across the board - I think that's something we can and should all agree on," Chief Executive Linda Yaccarino posted on Thursday.
"X has also been extremely clear about our efforts to combat antisemitism and discrimination," she added. "There's no place for it anywhere in the world - it's ugly and wrong."
Meanwhile, on Wednesday, Mr Musk responded affirmatively to a tweet that accused Jewish communities of pushing "hatred against whites" and which included anti-immigrant sentiments.
Media Matters said Mr Musk's tweet, which was seen by more than five million people, according to the site's statistics, amounted to an endorsement of an antisemitic conspiracy theory known as "white genocide". It argues that Jewish people systematically plot to encourage immigration of "non-white" people to Western countries in order to create demographic change and "eliminate" the white race.
The X owner denies he is antisemitic and later said his comments referred not to all Jewish people but to groups like the Anti-Defamation League (ADL) and other unspecified "Jewish communities".
In response to Musk's tweet, ADL Chief Executive Jonathan Greenblatt posted: "At a time when antisemitism is exploding in America and surging around the world, it is indisputably dangerous to use one's influence to validate and promote antisemitic theories."
Mr Musk has on several occasions repeated conspiracy theories and has also criticised social media watchdogs - including the ADL and other groups - for criticising his content moderation changes at X.
The company says that it has stronger brand safety controls than other social networks and that hate speech and extremism has fallen on the platform despite large cuts to the company's safety team. Several outside groups disagree with the company's own assessment.
Earlier this year Mr Musk threatened to sue the ADL, claiming it was "trying to kill this platform by falsely accusing it & me of being anti-Semitic". He blames pressure groups for a sharp drop in advertising revenue since his takeover.
While he has not carried through with his threat against the ADL, the company has sued another research and campaign group, the Center for Countering Digital Hate.
Mr Musk was scheduled to speak at the Asia-Pacific Economic Cooperation summit in San Francisco on Thursday but cancelled, citing a schedule change.
