US Defence Secretary Jim Mattis on Sunday said America's vast military continues to run smoothly, even though President Donald Trump's first month has been marked by consecutive crises.

"Welcome to democracy," Mattis told reporters in Abu Dhabi during his first trip to the region as Pentagon chief.

"It's at times wildly contentious, it's at times quite sporting, but the bottom line is this is the best form of government we can come up with."

Mattis was responding to a question about a comment last week from Army General Raymond "Tony" Thomas, who said the US government was in "unbelievable turmoil."

Trump's first few weeks have been notable for a growing scandal over possible White House-Kremlin ties, a botched travel ban and controversial cabinet picks.

The new Pentagon chief said Thomas may have been taken out of context and, as far as he is concerned, everything is running smoothly from a military perspective.

"The military job is to hold the line, to hold the line and to hold the line, while our government sorts out the way ahead and our people speak," he said.

"We don't have any disarray inside the military and that's where my responsibility lies."

Military commanders in the region say they have so far seen little impact from the presidential transition.

Mattis also referred to Trump's continued diatribe against journalists and his branding of several media outlets as the "enemy of the American People."

"I have had some rather contentious times with the press," said Mattis, a former Marine general whose fiery war talk has landed him in hot water on several occasions.

"But the press as far as I am concerned are a constituency that we deal with, and I don't have any issues with the press myself," he told reporters.

While campaigning and since his election, Trump has blasted the media on a near-daily basis, dismissing stories he dislikes of being "fake news" and calling journalists dishonest.

He took his disdain to a new level Saturday, when he said several US news networks were "the enemy."

Mattis is well known for his pithy sayings but has also gotten in trouble for some of his comments while commanding in Iraq and Afghanistan.

On the eve of the Iraq invasion, he told Marines to: "Engage your brain before you engage your weapon."

He also famously -- and controversially -- said in regard to fighting the Taliban that it's "a hell of a lot of fun to shoot them."