The chairman of the US Joint Chiefs of Staff said Tuesday it is only a matter of a "very short time" before North Korea can strike the US with a nuclear weapon.

"We should assume today that North Korea has that capability and has the will to use that capability," said Marine Gen. Joe Dunford during a Senate hearing on his reappointment.

While there are still technical elements the regime must overcome, he said, "I view those as engineering solutions that will be developed over time."

"Whether it's three months or six months or 18 months, it is soon, and we ought to conduct ourselves as though it is just a matter of time, and a matter of very short time, before North Korea has that capability."

Dunford's assessment has been echoed by other US officials and experts, but it also comes at a time of heightened tensions over an escalating war of words between Washington and Pyongyang.

On Monday, North Korea's foreign minister accused President Donald Trump of declaring war on his country by recently tweeting the North's leader Kim Jong-un and his regime "won't be around much longer."

The minister declared Pyongyang's right to defend itself by shooting down approaching American bombers even in international air space.

Despite the rhetoric, Dunford said, there has been no change in North Korea's military posture.

"While the political space is clearly very charged right now, we haven't seen a change in the posture of North Korean forces. We watch that very carefully," he said.

The military has consciously avoided making statements that would "exacerbate the current crisis," Dunford added, before emphasizing he would not comment on remarks from the senior political leadership.

The general provided assurances that the military can protect Hawaii and the US mainland against an intercontinental ballistic missile, which Pyongyang tested twice in July.

On North Korea's threat to test a hydrogen bomb over the Pacific Ocean, he said it would be an "incredibly provocative thing" to do.

He also cited regime survival as the main motivation behind Kim's pursuit of nuclear ballistic missiles, not a desire to take over South Korea. (Yonhap)