France has expressed concern after US border agents read the contents of a visiting French space scientist's smartphone and deported him after accusing him of "hateful" messages against US policy.

Since Donald Trump returned to the White House in January, his government has cut federal research funding and sought to dismiss hundreds of federal workers pursuing health and climate research.

"I learned with concern that a French researcher who was heading to a conference near Houston was forbidden from entering US territory before being expelled," Higher Education Minister Philippe Baptiste told AFP on Wednesday.

"This measure was taken by the US authorities because the researcher's phone contained exchanges with colleagues and friends in which he expressed his political opinion on the policies of the Trump administration on research," he said.

A diplomatic source said a space scientist was stopped at the US border on 9 March, with officials combing through the contents of his work laptop and phone during a "random" security check.

US agents found messages about the treatment of scientists under the new US administration that "showed hatred towards Trump and could be qualified as terrorism", the same source said.

They seized his equipment and sent him back to Europe on 10 March, they said.

Another source with knowledge of the case said the scientist was accused of owning "hateful and conspiratorial messages" and was told the FBI would investigate, though they then dropped the case.

Baptiste emphasised the importance of "freedom of opinion, free research and academic freedoms" and said he would defend these as long as they were "within the limits of the law".

The French foreign ministry said that its consular services had been informed of the incident, and that the United States was "sovereign" in deciding who could enter and remain on its territory. But it said it "deplored the situation".

The US Embassy in Paris redirected an AFP request for comment to the US customs, but the latter did not immediately reply.

According to the American Civil Liberties Union (ACLU), the US government believes its border agents are entitled to examine the contents of people's electronic devices as part of random security checks.

Rights groups including the ACLU sued the US government in 2017 over the increasing use of warrantless searches of cellphones and computers at the border, saying they were "unconstitutional".

According to the ACLU, it won the case in an initial federal court ruling but it was later overturned on appeal in 2021, prompting it to ask the US Supreme Court to hear the case.

Baptiste this month urged French research institutions to consider welcoming scientists abandoning the United States because of Trump's funding cuts.