President Donald Trump's national security adviser, Army Gen. H.R. McMaster is out and John Bolton, a former US ambassador to the United Nations, is set to replace him.

Bolton frequently appears on Fox News and writes that he thinks North Korea is an imminent threat to the US and must be dealt with immediately.

He dismissed North Korea's recent push for talks and maintains that war is still a good option.

President Donald Trump's national security adviser, Army Gen. H.R. McMaster is out and John Bolton, a former US ambassador to the United Nations, is set to replace him.

In late February, amid a marked thaw in tensions between North Korea and South Korea during which the prospect of diplomacy looked brighter than ever, Bolton wrote an opinion piece in The Wall Street Journal called "The Legal Case for Striking North Korea First."

In the article, Bolton argued that North Korea had given the US no choice and must be attacked before it perfected its fleet of nuclear intercontinental ballistic missiles. In his article Bolton never mentioned South Korea, which is in range of North Korea's massive installation of hidden artillery guns.

Experts estimate that thousands would die in Seoul, South Korea, the capital of a democratic, loyal US ally, for every hour of fighting with North Korea.

"It is perfectly legitimate for the United States to respond to the current 'necessity' posed by North Korea's nuclear weapons by striking first," Bolton said to conclude his article.

After South Korean diplomats said North Korean leader Kim Jong Un had expressed willingness to give up his country's nuclear weapons, Bolton dismissed it as a trick.

"The only thing North Korea is serious about is getting deliverable nuclear weapons," he told Fox News. Bolton frequently appears on Fox, Trump's favorite news station, to talk about North Korea in his characteristically hawkish way.

Bolton's Twitter feed is a constant stream of reminders of links between North Korea's weapons programs and those in Syria and Iran.

Bolton believes, not without evidence, that North Korea could become an exporter of dangerous technologies that could threaten US lives.

Trump already had a North Korea hawk — Bolton is a super hawk

President Donald Trump with Army Lt. Gen. H.R. McMaster at Trump's Mar-a-Lago estate. AP

McMaster isn't exactly a dove on North Korea. McMaster is believed to have pushed the idea of striking North Korea, though perhaps in ways designed to prevent all-out war.

In November and December, persistent reports came out that Trump's inner circle was weighing such a "bloody nose" attack on North Korea. But by the new year, military and administration officials had started to pour cold water on the notion.

On Thursday, the commander of the US military in the Pacific dismissed the possibility of a limited strike, saying the US military was planning for all-out war or none at all.