Elon Musk, the billionaire businessman and head of the Department of Government Efficiency, called Democratic Sen. Mark Kelly a “traitor” in a post on X after Kelly posted that he had visited Ukraine over the weekend.

Kelly, in a thread on X Sunday night, posted photos of his visit to Ukraine and wrote that “Everyone wants this war to end, but any agreement has to protect Ukraine’s security and can’t be a giveaway to Putin.”

In a reply to the thread, Musk responded, “You are a traitor.”

Kelly, a former Navy pilot and astronaut, responded in a separate post on X.

“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,” wrote Kelly, whose recent trip marked his third visit to Ukraine since 2023.

The comments from Musk, one of President Donald Trump’s closest advisers, comes weeks after an explosive meeting between Trump and Ukrainian President Volodymyr Zelenskyy in the Oval Office that devolved into a shouting match. During the stunning exchange, Trump and Vice President JD Vance rebuked Zelenskyy for his handling of the war, falsely blaming the Ukrainian leader for a conflict that began when Russia’s Vladimir Putin launched a full-scale invasion.

After the meeting, Zelenskyy left without signing an agreement that would have given the U.S. access to Ukraine’s mineral resources, which the country had hoped would ensure the continued flow of U.S. military support as it battles Russia.

Trump’s administration has embarked on a dramatic pivot away from the “ironclad” backing of Ukraine practiced by former President Joe Biden’s administration. Trump has falsely blamed Ukraine for starting the war with Russia, called Zelenskyy a “dictator” and frozen military aid and intelligence support in a bid to force Ukraine into making concessions to Russia.

Kelly, along with other Democrats, have been critical of Trump’s approach with Ukraine.

“If we abandon our ally Ukraine, we will be viewed by other countries including our other allies as untrustworthy and in the future we shouldn’t expect their help,” Kelly posted to X.

Kelly and Musk have feuded in the past. When Musk attacked Danish astronaut Andreas Mogensen last month, calling him “an idiot,” Kelly and his brother Scott Kelly, also an astronaut, pushed back.

“Hey @ElonMusk, when you finally get the nerve to climb into a rocket ship, come talk to the three of us,” Kelly wrote.