"With great audience comes greater responsibility," the bloc's internal market commissioner Thierry Breton posted on X, along with the letter laying out Musk's obligations to combat illegal content and disinformation under EU law.

The European Union has an ongoing probe into X, formerly Twitter, under its landmark Digital Services Act (DSA) which requires digital firms to effectively police online content to protect users from harm.

"The DSA obligations apply without exceptions or discrimination to the moderation of the whole user community and content of X (including yourself as a user with over 190 million followers)," Breton wrote to Musk.

Breton said the warning was motivated by the "risk of amplification of potentially harmful content in the EU," citing Musk's upcoming Trump interview, and also referencing his recent inflammatory comments about far-right riots in the United Kingdom.

"We are monitoring the potential risks in the EU associated with the dissemination of content that may incite violence, hate and racism in conjunction with major political -- or societal -- events around the world," he wrote.

"My services and I will be extremely vigilant to any evidence that points to breaches of the DSA and will not hesitate to make full use of our toolbox, including by adopting interim measures, should it be warranted to protect EU citizens from serious harm," Breton wrote.

Tesla and SpaceX CEO Musk has emerged as a major voice in US politics but is accused of turning the platform formerly known as Twitter into a megaphone for right-wing conspiracy theories since acquiring it in 2022.

New analysis from the Center for Countering Digital Hate shows Musk's false or misleading claims about the US elections have been viewed nearly 1.2 billion times on X -- which has around 300 million daily users, a third of them in the EU.

- Sweeping laws -

EU regulators' wide-ranging probe into X is looking into the effectiveness of the platform's efforts to combat disinformation -- following major cuts to content moderation resources under Musk's leadership.

Breton travelled to California in spring last year to meet tech leaders including Musk and stress their obligations under the sweeping new DSA and its sister law, the Digital Markets Act (DMA).

Back then, Musk pledged to work to comply with the new laws -- but the tone has since soured, with the billionaire accusing Brussels of regulatory overreach.

Musk lashed out at Brussels last month, accusing the European Commission of offering X a "secret deal" to censor speech -- and vowing to go court over the matter.

Breton rejected the notion of such a deal, telling Musk: "Up to you to decide whether to offer commitments or not... See you (in court or not)."