US Defence Secretary James Mattis jokingly told new National Security Adviser John Bolton he'd heard he was "the devil incarnate" as he welcomed him to the Pentagon on Thursday.

“I’ve heard that you’re actually the devil incarnate and I wanted to meet you,” Mr Mattis said as the two men shook hands and walked past reporters.

Mr Bolton is a defence hawk who has advocated using military force against North Korea and Iran. President Donald Trump recently - and controversially - appointed him to replace HR McMaster.