US prepared for NK military option: Trump

US President Donald Trump has said the use of force against North Korea is a secondary option

United States President Donald Trump warned North Korea that any US military option would be "devastating" for Pyongyang, but said the use of force was not Washington's first option.

"We are totally prepared for the second option, not a preferred option," 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."

Bellicose statements by 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 Pyongyang conducted its sixth and most powerful nuclear test on September 3.

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

The assessment by Marine Corps General Joseph Dunford, chairman of the US Joint Chiefs of Staff, about Pyongyang's military stance was in contrast to a South Korean lawmaker who said Pyongyang 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," Dunford said.

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

Ri was reacting to Trump's Twitter comments that Kim and 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 Trump has said he will never allow.

South Korean President Moon Jae-in urged Kim Jong Un to resume military talks and reunions of families split by the 1950-53 Korean War to ease tension.

"Like I've said multiple times before, if North Korea stops its reckless choices, the table for talks and negotiations always remains open," Moon said.