President Donald Trump has reportedly soured on Secretary of Defense Jim Mattis, and is making more US military policy on his own.

Trump didn't consult Mattis when he withdrew from the Iran deal, dropped military exercises with South Korea, and ordered the creation of a new military branch.

A Pentagon spokesperson said it was silly to think Mattis wasn't in the loop.

But the Pentagon has a history of disjointed responses to snap decisions from Trump.

President Donald Trump has reportedly soured on Secretary of Defense Jim Mattis and is now increasingly making US military policy on his own.

Trump withdrew from the Iran deal, stopped US military exercises in South Korea, and aired plans to create the first new military branch in 70 years all in snap, shock decisions that saw Mattis scrambling to keep up, according to a new report from NBC.

"They don't really see eye to eye," said a former senior White House official.

These policy moves represent a budding trend of Trump steering US foreign and military policy by himself, and often against the advice of his top advisers like Mattis, sources told NBC.

Trump has made a number of snap decisions that have met with slow responses from Mattis and the Pentagon.

In July 2017, Trump announced a ban on transgender people joining the US military. Four federal courts struck down the order, and by the end of the year Mattis was accepting transgender troops again.

Trump, unsatisfied with the Pentagon slow-walking his policy decisions, has increasingly cut out Mattis and taken matters into his own hands, NBC cites sources as saying.

Trump recently asked the military to come to the rescue of another imperiled policy by asking the National Guard to come to the border. Mattis reportedly resisted the idea in private but remained tight-lipped and dutiful in his public comments.

"[Mattis] didn't feel like the mission was well-defined," a senior White House official told NBC.

Now those National Guard soldiers are reportedly shoveling manure and feeding border patrol horses, among other menial tasks.

A Pentagon spokesperson told NBC that it was "silly" to say Mattis had been out of the loop on major US military policy decisions taken by Trump, but the White House and Pentagon have a record of making disjointed statements.

On Monday, Mattis is in Asia to lay ou the US's stance on China's militarization of the South China Sea, one of the few military issues on which Trump has yet to truly leave his mark.