US President Donald Trump has warned North Korea that any US military option would be "devastating".

However, he said the use of force was not his first option to deal with the country's ballistic and nuclear weapons program.

"We are totally prepared for the second option, not a preferred option," Mr Trump said at a White House news conference, referring to military force.

"But if we take that option, it will be devastating, I can tell you that, devastating for North Korea. That's called the military option. If we have to take it, we will."

Aggressive statements by Mr Trump and North Korean leader Kim Jong-un in recent weeks have created fears that a miscalculation could lead to action with untold ramifications, particularly since North Korea conducted its sixth and most powerful nuclear test on 3 September. 

Despite the increased tension, the United States has not detected any change in North Korea's military posture reflecting n increased threat, the top US military officer said today.

The assessment by Marine Corps General Joseph Dunford, chairman of the US Joint Chiefs of Staff, about North Korea’s military stance, was in contrast with a South Korean politician who said North Korea had boosted defences on its east coast.

"While the political space is clearly very charged right now, we haven't seen a change in the posture of North Korean forces, and we watch that very closely," Gen Dunford told a Senate Armed Services Committee hearing on his reappointment to his post.

In terms of a sense of urgency, "North Korea certainly poses the greatest threat today," Gen Dunford testified.

North Korean Foreign Minister Ri Yong Ho yesterday accused Mr Trump of declaring war on the North and threatened that North Korean forces would shoot down US warplanes flying near the peninsula after American bombers flew close to the Korean peninsula last Saturday.

Mr Ri was reacting to Mr Trump's Twitter comments that Mr Kim and Mr Ri "won't be around much longer" if they acted on their threats toward the United States.

North Korea has been working to develop nuclear-tipped missiles capable of hitting the US mainland, which Mr Trump has said he will never allow.

Gen Dunford said North Korea will have a nuclear-capable intercontinental ballistic missile "soon," and it was only a matter of a "very short time".

"We clearly have postured our forces to respond in the eventof a provocation or a conflict," the general said, adding that the United States has taken "all proper measures to protect our allies" including South Korean and Japan.

"It would be an incredibly provocative thing for them to conduct a nuclear test in the Pacific as they have suggested, and I think the North Korean people would have to realise how serious that would be, not only for the United States but for the international community," Gen Dunford said.

South Korean politician Lee Cheol-uoo, briefed by the country's spy agency, said North Korea was bolstering its defences by moving aircraft to its east coast and taking other measures after the flight by US bombers.

Mr Lee said the United States appeared to have disclosed the flight route intentionally because North Korea seemed to be unaware.

The United States has imposed sanctions on 26 people as part of its non-proliferation designations for North Korea and nine banks, including some with ties to China, the US Treasury Department's Office Of Foreign Assets Control Sanctions said this afternoon.

The US sanctions target people in North Korea and some North Korean nationals in China, Russia, Libya and Dubai, according to a list posted on the agency's website.