US President Donald Trump once left Prime Minister Narendra Modi shocked and concerned by telling him India and China didn't share a border, a new book by two Pulitzer-winning journalists claims.

"It's not like you've got China on your border," Trump told Modi, and the Indian premier's eyes "bulged out in surprise", Washington Post journalists Phillip Rucker and Carol Leonning report in A Very Stable Genius, according to the US newspaper.

The book's title is a shout-out to Trump's own description of his mental acumen.

....Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart. Crooked Hillary Clinton also played these cards very hard and, as everyone knows, went down in flames. I went from VERY successful businessman, to top T.V. Star..... Donald J. Trump (@realDonaldTrump) January 6, 2018

....to President of the United States (on my first try). I think that would qualify as not smart, but genius....and a very stable genius at that! Donald J. Trump (@realDonaldTrump) January 6, 2018

Rucker and Leonning write that Prime Minister Narendra Modi's expression "gradually shifted, from shock and concern to resignation" after Trump's remarks, and that one of the US president's aides felt Modi probably "left that meeting and said, 'This is not a serious man. I cannot count on this man as a partner'," the Post reports.

The aide told the book's authors that India "took a step back" in its diplomatic ties with Washington after that meeting.

"There is a scene where the president is talking with the prime minister of India and sort of gets his geography wrong and he says 'well at least you don't border China'... At times the president was dangerously uninformed" - @AshleyRParker w/ @NicolleDWallace pic.twitter.com/xdZr7iB1bb January 15, 2020

President Trump and Prime Minister Modi have met several times, and it isn't clear from the Post's report which of these meetings the authors described. India and the US are in talks for a Trump visit to India, possibly in February.

If confirmed, the India trip would be President Trump's first since he was voted to power in 2016.

Donald Trump's knowledge of the subcontinent's geography has come under scrutiny before. A TIME magazine correspondent wrote last year that the US president had to be corrected at a briefing where he said he knew Nepal and Bhutan were in India. He also reportedly "mispronounced Nepal as 'nipple' and laughingly referred to Bhutan as 'button'."