President Donald Trump speaks at the 2019 National Historically Black Colleges and Universities Week Conference in Washington, Tuesday, Sept. 10, 2019. (AP Photo/Carolyn Kaster)

World - US President Donald Trump has appointed Charlie Kupperman as acting national security adviser, replacing John Bolton.

Administration spokesman Hogan Gidley confirmed the appointment. "There is no one issue here...they just didn't align on many issues," Gildey told reporters outside the White House, according to broadcaster CSPAN.

Kupperman, a former Boeing and Lockheed Martin executive, was Bolton's deputy in the White House since the start of the year. 

The two worked together extensively over the years.

Trump said that he has asked Bolton to resign, citing strong differences of opinion with his hawkish assistant.

"I informed John Bolton last night that his services are no longer needed at the White House. I disagreed strongly with many of his suggestions, as did others in the Administration," Trump tweeted.

"I asked John for his resignation, which was given to me this morning," the president said, adding that he was aiming to announce a replacement next week.

Bolton seemed to take a parting shot at his boss, immediately responding on Twitter: "I offered to resign last night and President Trump said, 'Let's talk about it tomorrow.'"

Bolton also texted US media outlets to say he resigned on his own.

The move came as a surprise, with the White House putting out an advisory just an hour before the Trump announcement saying that Bolton would be taking part in a media briefing along with Secretary of State Mike Pompeo and Treasury Secretary Steven Mnuchin.

Bolton also continued to post administration policy on social media right up until the Trump announcement.

However, Trump has previously hinted at his disagreements with Bolton on foreign policy, and observers had regularly noted that the two made an odd couple, with the president more isolationist and the national security adviser an interventionist.

"John Bolton is absolutely a hawk. If it was up to him, he'd take on the whole world at one time," Trump told broadcaster NBC in June. "But that doesn't matter because I want both sides."

Bolton, Trump's third national security adviser, appeared to be waging numerous battles within the White House, most recently over Afghanistan and North Korea, that seemed at odds with the president's wishes.

Bolton was also behind the hard-line approach to countries such as Iran and Venezuela.

The national security advisor comes from the wing of the Republican Party more aligned with the policies of the George W Bush administration, in which he also served. He is often seen as a proponent and even an architect of the invasion of Iraq in 2003.

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