Grok faces an outcry after it emerged that users could sexualise images of women and children using simple text prompts such as "put her in a bikini" or "remove her clothes".

"In Europe, we will not tolerate unthinkable behaviour, such as digital undressing of women and children," said European Commission chief Ursula von der Leyen.

"It is simple -- we will not hand over consent and child protection to tech companies to violate and monetise. The harm caused by illegal images is very real," she said in a statement to AFP.

EU tech commissioner Henna Virkkunen said the probe would "determine whether X has met its legal obligations" under the bloc's Digital Services Act (DSA), which is designed to police internet giants.

She said the rights of women and children in the EU should not be "collateral damage" of X's services.

Brussels said it was investigating whether X had properly mitigated "risks related to the dissemination of illegal content in the EU, such as manipulated sexually explicit images, including content that may amount to child sexual abuse material".

The EU move comes despite repeated US threats of retaliation against enforcement of tech rules President Donald Trump's administration attacks as curbing free speech and unfairly targeting US firms.

- Three million images -

Under fire, Grok said earlier this month it was restricting image generation and editing to paying subscribers.

But that move did not stave off the EU's probe -- which follows investigations in multiple countries including France and the United Kingdom.

"Let's be very clear, child sexual abuse material is not a premium privilege," said EU commission spokesman Thomas Regnier.

According to research published Thursday by the Center for Countering Digital Hate, a nonprofit watchdog, Grok generated an estimated three million sexualised images of women and children in a matter of days.

As part of the new probe, the EU said it was widening an existing investigation into X aimed at tackling the spread of illegal content and information manipulation.

That move was due to the fact that X announced last week Grok will now power its recommendation system for users and EU concerns that the company had not adequately assessed the risks.

Contacted by AFP, X did not immediately react to the EU announcement.

Musk's social media platform, formerly known as Twitter, has been the target of an investigation under the EU's digital content rules since December 2023.

Brussels in December slapped a 120-million-euro ($140-million) fine on X -- the first-ever imposed under the DSA -- for violating transparency rules, triggering angry reactions from Trump's administration.

X's breaches included the deceptive design of its "blue checkmark" for supposedly verified accounts, and failure to provide access to public data for researchers.

Violators can in theory face much higher fines -- up to six percent of a provider's global turnover -- for breaching the DSA, and the EU has the power to ban offending platforms from Europe for repeated non-compliance.

The EU has insisted it will enforce its tech rules despite pressure from Washington -- which has pressed the bloc to roll them back.

The dispute over regulation comes as the EU has grappled with the Trump administration on multiple other fronts -- from the Ukraine war to trade to Greenland.

In outlining the new X probe, commission spokesman Regnier said the EU enforces its legislation "firmly, fairly, objectively" and did not "target any company based on its origin."

In a separate move, the European Commission on Monday added WhatsApp to its list of digital firms big enough to face stricter content rules under the DSA.

The Meta-owned service joins Facebook, TikTok, X and others in a list of 26 "very large online platforms," after its "channels" feature passed 45 million monthly active users in the European Union.

The tougher obligations will apply only to its "channels", considered a broadcasting feature, rather than its core messaging service.

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