Mike Pompeo speaks during a press conference in Washington. File photo

"I do not think the world properly knows just how close the India-Pakistan rivalry came to spilling over into a nuclear conflagration in February 2019," the likely future presidential contender wrote in "Never Give an Inch," his memoir of his time as Donald Trump's top diplomat and earlier CIA chief.

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Pompeo, who was in Hanoi for a summit between Trump and North Korean leader Kim Jong Un, said he was woken up with an urgent call from a senior Indian official.

"He believed the Pakistanis had begun to prepare their nuclear weapons for a strike. India, he informed me, was contemplating its own escalation," Pompeo wrote.

"I asked him to do nothing and give us a minute to sort things out," Pompeo said.

Pompeo said that US diplomats convinced both India and Pakistan that neither was preparing to go nuclear.

US diplomats convinced both India and Pakistan that neither was preparing to go nuclear.

"No other nation could have done what we did that night to avoid a horrible outcome," Pompeo wrote.