The breakfast meeting came amid US media reports that Mattis risks being isolated by Trump's more bellicose coterie of advisors, including Bolton, an Iraq War-era hawk who has advocated for military action in both Iran and North Korea.

Bolton "was here this morning," Pentagon spokeswoman Dana White said.

The two men decided to have "regular" meetings, she added, noting that CIA Director Mike Pompeo, Trump's pick to run the State Department, could join them.

When Mattis first met Bolton at the Pentagon last month, the defense secretary jokingly said: "I've heard that you're absolutely the devil incarnate and I wanted to meet you."

Mattis, a retired four-star Marine general, is one of a dwindling pool of original Trump picks not to have drawn negative attention from his mercurial boss.

According to multiple reports, after a suspected chemical attack in Syria this month, he successfully pushed Trump to only taking limited action in response, while Bolton wanted a larger operation.

Mattis used to meet regularly with Rex Tillerson, who was fired last month from his position as secretary of state.

Pompeo is seen as being more hawkish than Mattis, further raising the possibility of the Pentagon chief's influence waning.