WASHINGTON (AFP) - The United States is now closer than it has ever been to a nuclear war with North Korea, a former top US military officer warned on Sunday (Dec 31), saying he saw little prospect of a diplomatic solution.

Mike Mullen, a former chairman of the US joint chiefs of staff, attributed the rising danger to Donald Trump's "incredibly disruptive" presidency.

"And in my view, an incredibly dangerous climate exists out there in that uncertainty with how this all ends up," he said on ABC's This Week.

"One in particular that is top of the list is North Korea."

"We're actually closer, in my view, to a nuclear war with North Korea and in that region than we have ever been," he said, adding, "I don't see the opportunities to solve this diplomatically at this particular point."

Mullen, who served as the top US military adviser to both presidents George W. Bush and Barack Obama, questioned whether Trump can be constrained by US Defence Secretary Jim Mattis, National Security Adviser HR McMaster or White House chief of staff John Kelly.

"Will he follow through on his rhetoric? Or will we actually be able to get to a situation where it could be solved peacefully?" Mullen asked.

"I'm just more inclined to see over time that the rhetoric seems to be where the president is, and that will limit the constraining ability that both Jim Mattis and H.R. McMaster and John Kelly have," he said.

Trump warned in a speech to the UN General Assembly in September that the United States "will have no choice but to totally destroy North Korea" if forced to defend itself or its allies.

In August, he vowed to unleash "fire and fury" on North Korea if it continued to threaten the United States or its allies.

Pyongyang conducted its sixth nuclear test Sept 3 and, despite escalating sanctions, on Nov 28 it test-launched an intercontinental missile capable of striking the United States.

Republican Senator Lindsey Graham, who warned Sunday of a sharp increase in the risk of pre-emptive attack if North Korea conducts any more tests, said Trump had decided early on to deny North Korea the capability to strike the United States with a nuclear-tipped missile.

"2018 will be a year of opportunity and extreme danger," Graham said on CBS' Face The Nation.

"Now, the Iranians are watching the way he engages with North Korea, and vice versa. So we've got a chance here to deliver some fatal blows to really bad actors in 2018. But if we blink, God help us all."