British Prime Minister Theresa May on Sunday said she believed President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE was acting with the best interests of the U.S. in mind, as concerns are raised over his mental fitness.

“What I make of him is somebody who is taking decisions on what he believes is in the best interest of the United States,” May said in the interview with BBC's Andrew Marr.

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Her comments came as she was pressed by Marr about Trump's mental stability.

"In the States there are quite serious questions being raised by some people about his mental state. Do you think they're serious?" Marr asked.

May seemed to dismiss the question, and reiterated her view that Trump has the best interests of the U.S. in mind.

The interview came amid growing fears over the president's mental fitness following the release of a new book containing numerous damaging allegations about Trump.

The president's recent tweets, including one in which he compared the size of his "nuclear button" to North Korean leader Kim Jong Un's, have also spurred concerns.

The White House pushed back against those fears, saying it was "disgraceful and laughable" to suggest Trump is unfit for office.

Trump himself also weighed in on the matter, tweeting on Saturday: “Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart."

During her interview on Sunday, May also confirmed that Trump will visit the United Kingdom (U.K.).

“He will be coming to this country,” May told Marr.

May’s office last year reportedly refused to revoke an invitation for Trump to visit the U.K. after he retweeted videos posted by a British far-right nationalist leader.