By Foo Yun Chee and Sheila Dang

BRUSSELS/DALLAS (Reuters) -EU industry chief Thierry Breton on Thursday opened an investigation into Elon Musk's X, the first under new EU tech rules, after earlier reprimanding the social media platform, TikTok and Meta for not doing enough to tackle the spread of disinformation following Hamas' attack on Israel.

All three platforms have seen a surge of false content about the Israel and Hamas conflict, with disinformation appearing to be most prevalent on X, social media researchers told Reuters.

Breton's move ramps up the pressure on TikTok and Meta to remove illegal and harmful content from their platforms in order to comply with the Digital Services Act (DSA).

The DSA, which entered into force in November last year, forces very large online platforms and search engines to do more to tackle illegal content and risks to public security, and protect their services against manipulative techniques.

X CEO Linda Yaccarino said earlier on Thursday the platform had removed hundreds of Hamas-affiliated accounts and taken action to remove or label tens of thousands of pieces of content since the attack, in response to a letter from Breton.

"We have sent @X a formal request for information, a first step in our investigation to determine compliance with the DSA," Breton said in a posting on X.

X declined to comment.

It has until Oct. 18 to provide details on how its crisis response protocol is activated and functions, and until Oct. 31 on other issues.

A move by Musk to cut off free academic access to a data tool earlier this year is making it more challenging to track keywords and hashtags, forcing researchers to manually sift through content to trace disinformation, researchers said.

Since taking over Twitter, Musk has slashed the workforce to roughly 1,500 from 7,500 employees to cut costs, including many who worked on content moderation, identifying and taking down coordinated propaganda campaigns and curating reliable content.

X has also lost two heads of trust and safety and one head of brand safety, who worked to prevent ads from appearing next to harmful content. The company risks fines of as much as 6% of its global turnover if found guilty of DSA violations.

The Frenchman earlier on Thursday gave TikTok CEO Shou Zi Chew 24 hours to step up efforts to remove illegal and harmful content from the short video app.

Breton's warning in a letter to Chew, first seen by Reuters, follows similar letters to X, formerly Twitter, owner Musk and Meta Platforms' Mark Zuckerberg earlier this week. Breton subsequently posted the letter on social media platform Bluesky.

Breton said in the letter to TikTok, owned by Chinese conglomerate ByteDance, that he had indications that it was being used to disseminate illegal content and disinformation in the EU after the Hamas attacks.

"Given that your platform is extensively used by children and teenagers, you have a particular obligation to protect them from violent content depicting hostage taking and other graphic videos which are reportedly widely circulating on your platform without appropriate safeguards," he said.

(Reporting by Foo Yun Chee; Writing by Philip Blenkinsop; Editing by Jane Merriman, Elaine Hardcastle, Jan Harvey and Sandra Maler)