Victor Cha, a Korean American former White House official, had clashed with the Trump administration over a proposed preventive strike on North Korea, claiming it could do more harm than good.
The president and his top team has reportedly been considering a limited strike on the rogue state, as the latest escalation in an international dispute over Kim Jong-un’s nuclear programme.
But Mr Cha categorically voiced his opposition to the move, warning it risked provoking dictator Kim into launching a nuclear strike that could kill millions of Americans.
He wrote: “Force will be necessary to deal with North Korea if it attacks first, but not through a preventive strike that could start a nuclear war.”
A preventive strike, experts besides Mr Cha also warned, could lead to catastrophic retaliation not only against the US, but also against its ally, South Korea.
However, the administration appears to be unhappy about the difference of opinion and has now ruled out Mr Cha as a candidate for the prestigious position.
A US official confirmed: “The White House has moved on to other potential candidates.”
Experts reacted with surprise and alarm to the revelation that Mr Cha was no longer in contention for the position.
Tom Wright, a senior foreign policy fellow at Brookings Institution, said: “This is a huge story. And extremely worrying.
“I thought the Trump administration was bluffing on a preventive strike… but the news about Cha no longer being nominated is the type of costly signalling that convinces me I may have been wrong. They are seriously considering it.”
It comes amid fears North Korea is not far off being capable of striking US shores with its newly-developed nuclear and ballistic missiles.
In his State of the Union address on Tuesday, Mr Trump vowed the US would continue to exert “maximum pressure” to prevent Kim from making further nuclear advances that could threaten his country.
He told Congress: “No regime has oppressed its own citizens more totally or brutally than the cruel dictatorship in North Korea.
“North Korea’s reckless pursuit of nuclear missiles could very soon threaten our homeland. We are waging a campaign of maximum pressure to prevent that from ever happening.
“Past experience has taught us that complacency and concessions only invite aggression and provocation.”
It was thought that tensions in the Korean peninsula had cooled somewhat after North and South Korea came to an agreement allowing Kim Jong-un’s athletes to compete in the Winter Olympic Games in the South.
However Mr Trump’s rhetoric continues to characterise North Korea as an enemy threat, hinting at further escalation in 2018.