WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Tuesday he had no reservations or concerns about President Donald Trump’s incoming national security adviser, John Bolton, a hawk who has advocated using military force against North Korea and Iran.

FILE PHOTO - Former U.S. Ambassador to the United Nations John Bolton speaks at the Conservative Political Action Conference (CPAC) in Oxon Hill, Maryland, U.S. February 24, 2017. REUTERS/Joshua Roberts/File Photo

Amid speculation the two men will clash on a host of major national issues, Mattis said he would meet Bolton for the first time later this week at the Pentagon with the goal of forging a partnership.

“We’re going to sit down together (this week), and I look forward to working with him. No reservations. No concerns at all,” Mattis told a group of reporters at an impromptu briefing.

“Last time I checked, he’s an American and I can work with an American. Okay? I’m not the least bit concerned with that sort of thing.”

Trump has shaken up his core national security team in the past two weeks, replacing National Security Adviser H.R. McMaster and firing Rex Tillerson as his secretary of state.

The moves within a small group of just a handful of advisers have raised questions about whether Mattis could find himself increasingly isolated in his views and outmaneuvered by Bolton, an inveterate bureaucratic infighter whose 2007 memoir is titled: “Surrender Is Not An Option.”

Mattis had forged a close relationship with both McMaster and Tillerson as he successfully advocated to keep U.S. troops in Afghanistan and strengthen ties with NATO, despite Trump’s skepticism about both the 16-year-old war and the trans-Atlantic alliance supporting it.

Warning about the horrors of a war on the Korean peninsula, Mattis has also promoted a diplomatically-led strategy to pressure North Korea over its efforts to build a nuclear-tipped missile capable of striking the United States.

CAUTIOUS COMMUNICATOR

Mattis has also been a cautious communicator.

After Trump announced plans to talk with North Korean leader Kim Jong Un, Mattis was so concerned he might say something to upset the process that the defense secretary opted earlier this month to stop making any substantive public remarks about North Korea at all.

“Right now every word is going to be nuanced and parsed apart across different cultures, at different times of the day, in different contexts,” Mattis said at the time.

On the other hand, Bolton, a 69-year-old Fox News analyst and former U.S. ambassador to the United Nations, in the past has called for regime change in North Korea and has previously been rejected as a negotiating partner by Pyongyang.

In 2003, on the eve of six-nation talks over Pyongyang’s nuclear program, he lambasted then-North Korean leader Kim Jong Il in a speech in Seoul, calling him a “tyrannical dictator.” North Korea responded by calling Bolton “human scum.”

More recently, Bolton described Trump’s plan to meet North Korean leader Kim Jong Un as “diplomatic shock and awe” and said it would be an opportunity to deliver a threat of military action.

Bolton has been downplaying his aggressive rhetoric in his initial conversations with some current and former U.S. officials, and sought guidance on how to approach Mattis, sources familiar with those conversations told Reuters.

Barry Pavel, a U.S. national security expert at the Atlantic Council think-tank, said it was too soon to predict Bolton’s style or draw conclusions about how he would run the National Security Council.

“When you’re in a position like he’s going into, it’s a very, very solemn set of responsibilities ... and those have a restraining factor,” Pavel said.

Asked by Reuters about the split between his world views and Bolton’s, Mattis sought to dismiss concerns, suggesting lively debate would help ensure Trump has a wide array of options.

“Well, I hope that there’s some different world views. That’s the normal thing you want unless you want groupthink,” Mattis said.

“You know, don’t worry about that. We’ll be fine.”