The EU has warned Elon Musk that X is being used to spread "disinformation" after Hamas' attack on Israel.
In a post on the site formerly known as Twitter, the bloc's industry chief said "violent and terrorist content" had not been taken down, despite warnings - as is required by EU law.
Mr Musk said his company had taken action, including by removing newly-created Hamas-affiliated accounts.
He asked the EU to list the alleged violations.
Thierry Breton, Commissioner for the Internal Market of the European Union, did not give details on the disinformation he was referring to in his letter to Mr Musk.
However, he said that instances of "fake and manipulated images and facts" were widely reported on the social media platform.
"I therefore invite you to urgently ensure that your systems are effective, and report on the crisis measures taken to my team," he wrote in his letter which he shared on social media.
His letter comes days after the Palestinian militant group Hamas launched an attack on Israel, killing hundreds of residents and taking dozens of hostages.
In response, Israeli forces have launched waves of missile strikes on Gaza which have killed more than 900 people.
In his response on X, Mr Musk said: "Our policy is that everything is open and transparent, an approach that I know the EU supports.
"Please list the violations you allude to on X, so that the public can see them."
Mr Breton said that Mr Musk was "well aware of your users' - and authorities' - reports on fake content and glorification of violence", adding that it was up to him to "demonstrate that you walk the talk".
The EU Digital Services Act (DSA) is designed to protect users of big tech platforms.
It became law last November but firms were given time to make sure their systems complied.
On 25 April, the commission named the very large online platforms - those with over 45 million EU users - that would be
subject to the toughest rules, among them X. The law came into effect four months later in August.
Under the tougher rules, larger firms have to assess potential risks they may cause, report that assessment and put in place measures to deal with the problem.
Failure to comply with the DSA can result in EU fines of as much as 6% of a company's global turnover, or potentially suspension of the service.
Mr Musk dissolved Twitter's Trust and Safety Council shortly after acquiring the company in 2022. Formed in 2016, the volunteer council contained about 100 independent groups who advised on issues such as self-harm, child abuse and hate speech.
