President Donald Trump plans to nominate Admiral Harry Harris, commander of Pacific Command, to be the ambassador to South Korea, NBC News reported citing sources familiar with the situation.

Harris had originally been the president's pick for ambassador to Australia, a key U.S. ally. His confirmation hearing for that post was originally scheduled for Tuesday, but it was postponed. He was widely expected to be confirmed.

In his 2014 confirmation hearing to be commander of Pacific Command, Harris said he believed North Korea to be the "most volatile and dangerous threat" in the region.

Last year, before the House Armed Services Committee, Harris said North Korea's continued success in nuclear tests will only spur Kim Jong Un further down "this reckless and dangerous path."

But the admiral emphasized that in dealing with North Korea, the U.S. must "be guided by a strong sense of resolve both publicly and privately in order to bring Kim Jong Un to his senses, not his knees."

Harris made some waves in late 2016, when he sided with Trump on a controversial issue. On the 75th anniversary of Pearl Harbor, Harris opened his remarks by taking a dig at Colin Kaepernick and other NFL players who take a knee during the national anthem.

"You can bet that the men and women that we honor today and those who died that fateful morning 75 years ago never took a knee and never failed to stand when they heard our national anthem being played," Harris said.

The news comes after the Senate Foreign Relations committee on Monday endorsed Mike Pompeo for secretary of State.

The role of ambassador to South Korea was one of many vacancies in the State Department amid negotiations for a summit between Trump and Kim.

— CNBC's Amanda Macias contributed to this report.