US defence secretary Jim Mattis has said the diplomatic push to solve the North Korean nuclear weapons crisis was at such a delicate stage that he will not publicly discuss the talks.

The US is not ready to collaborate militarily with Russia, defence secretary Jim Mattis said, appearing to close the door for now on any effort to work more closely with Moscow in the fight against Islamic State militants in Syria.

Jim Mattis has told Nato allies that they must start increasing defence spending by the end of the year or the Trump administration will "moderate its commitment" to them.