WASHINGTON (Reuters) - Victor Cha, a former White House official who had been the Trump administration’s choice to be the next U.S. ambassador to South Korea, is no longer being considered for the post, U.S. officials said on Tuesday.

The Washington Post quoted people familiar with the matter as saying Cha, a Korean-American, had raised concerns with White House officials over their consideration of a risky limited strike on North Korea and about the administration’s threats to tear up a bilateral trade deal with Seoul.

One U.S. official acknowledged to Reuters that there had been policy disagreements, but did not offer specifics.

In an article in the Washington Post on Tuesday evening, Cha, who for months had been expected to be nominated for the sensitive diplomatic post in a major ally that has been vacant for over a year, made clear he was no longer being considered.

He said a preventative military strike was not the way to deal with the crisis over North Korea’s development of nuclear missiles capable of hitting the United States, saying it carried a huge risk of escalating into a war that could kill hundreds of thousands of Americans.

“Force will be necessary to deal with a North Korea if it attacks first, but not through a preventative strike that could start a nuclear war,” Cha wrote.

Cha is a former director for Asian affairs on the White House National Security Council and served as deputy head of the U.S. delegation in talks with North Korea over its nuclear program during the George W. Bush administration.

“The White House has moved on to other potential candidates,” another U.S. official said, adding that the administration was looking for the “right person” to nominate. The official declined to give the reason for Cha being dropped from consideration.

News that Cha was out of the running came just ahead of U.S. President Donald Trump’s first State of the Union speech on Tuesday night, during which he was expected to address North Korea, his most pressing foreign policy challenge.

The administration has said all options are on the table in dealing with North Korea and officials say Trump and his advisers have discussed the possibility of a limited “bloody nose” strike, alarming experts who warn that this could trigger catastrophic retaliation, especially on South Korea.

Officials have repeated stressed the administration prefers a diplomatic solution and debate on military options has lost some momentum in recent weeks after North and South Korea resumed talks ahead of next month’s Winter Olympics to be held in the South.

In July, Cha called for a new approach to diplomacy toward North Korea, arguing that China must be a central part of future negotiations and should pay for Pyongyang to halt and roll back its nuclear and missile programs. (wapo.st/2x2vCgL)

In recent tweets and retweets on Twitter, Cha has highlighted the administration’s policy on tougher sanctions and articles stressing the importance of diplomacy.

Experts reacted with concern on Twitter to Tuesday’s news.

“This is a huge story. And extremely worrying,” said Tom Wright, a senior foreign policy fellow at the Brookings Institution.

“For months I thought Trump admin was bluffing on a preventive strike...but the news about Victor Cha no longer being nominated is the type of costly signaling that convinces me I may have been wrong. They are seriously considering it.”

Abraham Denmark, a former deputy assistant secretary of defense for East Asia under President Barack Obama, expressed astonishment.

“Wow. To drop an Ambassador nomination for a major treaty ally in the midst of a major crisis is unprecedented, and borders on strategic malpractice.”

The administration’s failure to fill the post and a number of other key foreign policy positions has brought criticism in Congress and among former U.S. official and experts.

South Korea’s Yonhap news agency reported in December that the administration had formally requested South Korea’s approval for the nomination.

South Korean officials had said they were hoping Cha would be in position in time for the Olympics, which will be attended by U.S. Vice President Mike Pence.