Washington (CNN) Speaking publicly for the first time about President Donald Trump's newly named national security adviser, John Bolton , Secretary of Defense James Mattis said Tuesday that he looked forward to working with the former UN ambassador and that he hoped the two men held "different world views" to avoid "group think".

"I'll tell you right up front, it's going to be a partnership, we are going to go forward," Mattis told reporters at the Pentagon.

Asked about his and the former UN ambassador's differing world views, Mattis responded, "I hope that there's some different world views. That's the normal thing you want unless you want group think."

Despite both having served in senior roles in government, the two men have never met, something Mattis acknowledged Tuesday, saying that Bolton was going to come over to the Pentagon this week for discussions.

"We are going to sit down together and I look forward to working with him," Mattis said.

A senior defense official told CNN that the Pentagon does not expect any conflict with Bolton because the job of national security adviser is more of an "aggregator" than that of a "policy adviser."

"We don't see actual changes in policy" coming down the road just because Bolton is on the job, the official said.

Mattis also stressed that he had no concerns regarding Bolton's appointment.

"No reservations, no concerns at all. Last time I checked he's an American. I can work with an American, OK? So I'm not the least bit concerned with that sort of thing," Mattis said.

Trump team changes

Mattis also acknowledged other recent changes made by Trump, including his decision to fire Secretary of State Rex Tillerson, but argued that the personnel changes were having no impact on preparations for the potential summit with North Korea or the administration's response to Russia's alleged use of a chemical nerve agent in the United Kingdom.

He said preparations for the North Korean summit were "on track" and that there has been a "very methodical process putting together with our allies, Japan and ROK, Republic of Korea, what the talking points look like there, the discussion points about what we're going to negotiate over, that normal consultation has been going on here, there are other nations also involved."

Mattis said the State Department is leading the diplomatic effort on North Korea despite the recent departure of Tillerson.

"It's all on track. As it has been all along, it is diplomatically led," Mattis said, adding that the acting secretary of state, Deputy Secretary John Sullivan, was capable of carrying out the necessary preparations.

"People who actually do the crafting and that sort of thing, who've got the guidance and that's all ongoing, these are the people who roll up their sleeves and do the work now. They haven't changed at all. It's very steady, there have been obviously changes in the national security structure right now but it's had no effect here at the Pentagon and there's been as you can see no significant change in the policy as we go forward," Mattis said.

Mattis pointed to the recent expulsion of 60 Russian diplomats and closure of the Russian consulate in Seattle in response to the attempted assassination in the UK as evidence that the administration was continuing to carry out policy despite the change in personnel.

"We have very strong institutions in this country and they continue to do the work," he said.

Reaction to Russia

Mattis also had tough words for Russia and its president, Vladimir Putin.

"Russia has chosen to be a strategic competitor even to the point of reckless activity and that's the only thing that it can be called to the innocent people in Salisbury who were exposed possibly to the potential of being murdered by this stuff," Mattis said.

Asked if Putin was responsible for Russia's actions in Salisbury as well at its military intervention in Crimea and armed support of separatists in eastern Ukraine, Mattis said, "Certainly he is responsible as the head of state."

But Mattis did say that high-level contacts between the US and Russian militaries did manage to prevent a clash in Syria this week after Russia was able to get pro-regime Russian mercenaries that had been amassing near US troops in Syria to pull back.

Mattis acknowledged for the first time that Russian mercenaries had participated in a February attack on US troops and their local Syrian allies, an attack that left over 100 pro-regime fighters and multiple Russians dead, later acknowledging that Russia had at least some control over these groups after Russia helped get similar fighters to pull back from positions that were close to US forces in Syria.

Mattis said that Russia was able to get the mercenaries to pull back, following high level talks between the Chairman of the Joint Chiefs, Gen. Joseph Dunford and his Russian counterpart, Gen. Valery Gerasimov, which he said helped avert another clash like the one last month.

"These were forces moving into more advanced positions, too close, de-confliction discussions with -- between our chairman and his Russian counterpart, General Gerasimov, and those elements fell back so we have also drawn off slightly in order to maintain the de-confliction between the elements there. So it looks like this time it was resolved through the de-confliction communication line, it did not go into harm's way as it did there a month ago," Mattis said of the pro-regime forces, which he described as being similar to the Russian mercenaries that were killed in February.

A coalition military official told CNN earlier on Tuesday that the pro-regime forces had begun to pull back west across the Euphrates River, which serves as a demarcation line between Russian and US-backed forces in Syria. Those same pro-regime forces had previously been massing and taking up positions east of the river, an area that is nearby where US troops are advising local Syrian Democratic Forces fighting ISIS. The official said that the pro-regime forces have now taken up positions west of the river but said the coalition would continue to observe them.

Dunford and Gerasimov discussed Syria last week and, while calls between the two top generals are rare, it was the second such conversation in less than two weeks.

"This is a recent development but we think that the potential for a clash there, thanks to the Russian direction to this group, has been reduced. Obviously you can't remove anything from that very complex zone but it looks like it went in a stable direction for right now," Mattis said.

Asked if the Russians' ability to get the mercenaries to pull back showed that Russia is in control over these types of forces, Mattis responded, "I believe they are."