Washington (CNN) A former candidate to be the Trump administration's ambassador to South Korea has issued a direct challenge to the White House over its consideration of a pre-emptive strike against North Korea.

The long rumored candidate to be US ambassador to South Korea, Victor Cha, will no longer be nominated, according to a senior administration official and a person familiar with the situation. The reversal, sources tell CNN, was driven by a disagreement over a "bloody nose" strike against Pyongyang.

Cha, a widely respected academic and former Bush administration official, said in a Washington Post op-ed Monday that the answer to the "real and unprecedented threats" North Korea presents is not, "as some Trump administration officials have suggested, a preventive military strike."

Instead, Cha laid out what he called "a forceful military option available that can address the threat without escalating into a war that would likely kill tens, if not hundreds, of thousands of Americans."

The prospect of a "bloody nose" strike against North Korea has split the Trump administration' s most senior national security staff, sources tell CNN, with military leaders -- including Chairman of the Joint Chiefs of Staff Gen. John Dunford, Defense Secretary James Mattis and Secretary of State Rex Tillerson -- expressing concern about the prospect.

Sources tell CNN that Cha's op-ed was meant as a direct message to advocates of a possible strike inside the National Security Council, including national security adviser H.R. McMaster.

One person familiar with the tug of war over Cha's candidacy told CNN that the decision to pull his name from contention was partly an effort to avoid Cha becoming a pawn in the spat between the White House, State Department and Pentagon about the wisdom of a "bloody nose" strike.

JUST WATCHED Source: Trump to give 'eye-opening' remarks on NK Replay More Videos ... MUST WATCH Source: Trump to give 'eye-opening' remarks on NK 01:24

Multiple White House officials did not respond to requests for comment. The Washington Post was first to report that Cha's nomination was no longer expected.

The policy difference over a pre-emptive strike drove the White House decision not to formally nominate Cha to be ambassador to South Korea, sources tell CNN, leaving the position unfilled even as tensions between the Trump administration and Pyongyang have raised fears of a nuclear confrontation.

Speaking during his first State of the Union address to Congress on Tuesday, President Donald Trump highlighted the dangers Pyongyang poses.

"North Korea's reckless pursuit of nuclear missiles could soon threaten our homeland," Trump warned. He added that "past experience has taught us that complacency and concessions only invite aggression and provocation. I will not repeat the mistakes of past administrations that got us into this dangerous position."

'The nuclear threat it could pose'

"We need only look at the depraved character of the North Korean regime to understand the nature of the nuclear threat it could pose to America and our allies," Trump added.

At a time of mounting worries about a possible nuclear confrontation with North Korea, the delay in filling the crucial post of a US ambassador to Seoul is confounding, observers say. It is also potentially damaging to US security interests at a time when understanding nuances on the Korean Peninsula -- home to more than 28,000 US troops -- is critical.

CNN previously reported that the administration had sent Cha's name to Seoul in December, where he received swift approval, two sources familiar with the matter said.

That process -- almost always a quick, rubber-stamp affair -- happens only after candidates have received full security clearance and gotten signoff from the White House.

Cha had even been escorted through the West Wing several months ago to meet people, including senior staff, according to an official and a source familiar with the matter.

JUST WATCHED Leaders warn against strike on North Korea Replay More Videos ... MUST WATCH Leaders warn against strike on North Korea 02:32

Democratic Sen. Tammy Duckworth of Illinois said the failure to put an ambassador in South Korea sent a terrible message to US allies in the region, and made the jobs of US diplomats and troops in Seoul much harder.

"I've been ringing the alarm bells about Donald Trump recklessly pushing us toward war with North Korea for months, and this is yet another reason for every American to worry," Duckworth told CNN. "If this report is true and we've truly reached a point where opposing unprovoked armed conflict is a disqualifier from serving as ambassador -- a role literally intended to prevent war -- in one of the most tenuous regions of the world, then this represents a troubling development and a setback for diplomacy."

Professional diplomats stress that an ambassador provides insight and intelligence that is much harder to glean from Washington.

"South Korea is important -- you really need somebody monitoring it closely -- and nobody will have the access an ambassador has," said Ronald E. Neumann, president of the American Academy of Diplomacy and a former ambassador to Algeria, Bahrain and Afghanistan. "You can't just keep tabs on where that's going and how it affects policy from Washington."

Intense disagreements

Cha was asked by the White House whether, as ambassador, he could help support the diplomatic efforts that would surround a pre-emptive strike. He expressed reservations about such a move, a source familiar with the matter said.

After he voiced those concerns, the White House went mostly silent, even as the South Koreans were in the process of approving his nomination in the process known as agrément.

The White House finally informed Cha over the weekend that his nomination wasn't moving forward, but provided no explanation why.

JUST WATCHED Why State Department vacancies are a big deal Replay More Videos ... MUST WATCH Why State Department vacancies are a big deal 03:18

What's not clear is whether the White House can find a viable candidate who would check all its required boxes, including support for a pre-emptive strike, backing a harsher trade policy and having expertise in the Korean Peninsula.

Cha, a highly regarded former National Security Council adviser, scholar and prize winning author on Asian affairs, served as director for Asian affairs on President George W. Bush's National Security Council.

As deputy head of the US delegation to multinational talks with North Korea that started in 2003, he has firsthand experience navigating the shoals of negotiations with Pyongyang.

Cha now teaches at Georgetown University, runs the Asian studies program within its school of foreign service and is a senior adviser at the Center for Strategic and International Studies.