A US senator has hit back at Elon Musk, saying the billionaire Trump adviser is “not a serious guy” after he branded the politician a “traitor” for visiting Ukraine.

Mark Kelly, the Democrat senator for Arizona and a former US navy combat pilot, made his third visit to Kyiv since 2023 earlier this month, meeting with wounded service members and Ukrainian officials.

Posting on X after he left the country, he said “what I saw proved to me we can’t give up on the Ukrainian people”, and in a break from US President Donald Trump‘s stance on negotiations, called for security guarantees in any peace deal.

“Everyone wants this war to end, but any agreement has to protect Ukraine’s security and can’t be a giveaway to Putin,” he said.

Mr Musk, who also leads the Department of Government Efficiency (DOGE), an advisory department under the Trump administration, replied on X and said: “You are a traitor.”

Firing back at the Tesla chief, Mr Kelly said: “Traitor? Elon, if you don’t understand that defending freedom is a basic tenet of what makes America great and keeps us safe, maybe you should leave it to those of us who do.”

He then told reporters back in the US the billionaire is “obviously not a serious guy”.

It comes as Ukraine’s President Volodymyr Zelenskyy arrives in Saudi Arabia ahead of his team having talks with America’s top diplomat on Tuesday.

Mr Zelenskyy will not be at the meeting with US secretary of state Marco Rubio, but his team will try to improve relations following his disastrous 28 February visit to Washington, which descended into an Oval Office argument with the US president and vice president JD Vance.

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It also comes as Mr Musk said X was hit by a “massive cyber attack” on Monday, with thousands of users reporting they were unable to use the platform.

Later, he claimed in an interview with Fox News that IP addresses involved in the cyber attack were traced to locations “in the Ukraine area”.