Washington DC - Far-right billionaire Elon Musk sparked outrage by Senator Mark Kelly a "traitor" on Monday after the Arizona Democrat visited Ukraine and said it was important that Americans stand with the war-torn country.

Kelly, a former NASA astronaut and US Navy pilot, was in Ukraine – which has been battling a Russian invasion for more than three years – over the weekend.

His trip comes at a time when President Donald Trump is seeking a quick end to the conflict by pivoting to Moscow and pausing US military aid to Kyiv.

"Just left Ukraine. What I saw proved to me we can't give up on the Ukrainian people. Everyone wants this war to end, but any agreement has to protect Ukraine's security and can't be a giveaway to (Russian President Vladimir) Putin," Kelly said on X.

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"Donald Trump is trying to weaken Ukraine's hand and we are owed an explanation," Kelly added in another post about his visit.

Musk, the world's richest person and Trump's top campaign donor, responded to Kelly on X by saying: "You are a traitor."

Kelly hit back, writing: "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."

Ukraine and the US began talks in Saudi Arabia on Tuesday with a partial ceasefire with Russia on the table, after Trump re-established contact with Putin and ramped up pressure on Ukraine's President Volodymyr Zelensky to end the war – without offering the security guarantees Kyiv wants.

Kelly later described SpaceX boss Musk to reporters as "not a serious guy. He should go back to building rockets," according to NBC.

California Senator Adam Schiff threw his support behind fellow Democrat Kelly.

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"Mark Kelly is a retired Navy pilot, astronaut, and dedicated public servant. He is a hero. Elon Musk is a self-absorbed coward. Hope that clears things up," Schiff said on X.

Republican congressman Don Bacon also weighed in.

"It's not decent, it's not right... I don't agree with Senator Kelly on some things, but we don't call each other a traitor, and I think it reflects bad on the White House," he told CNN correspondent Manu Raju.