Donald Trump has issued fresh threats of swift and forceful retaliation against nuclear North Korea, declaring the US military “locked and loaded” and warning that the communist country’s leader “will regret it fast” if he takes any action against American territories or allies.

The warnings came in a cascade of unscripted statements throughout the day, each ratcheting up a rhetorical stand-off between the two nuclear nations.

The US president appeared to draw another red line that would trigger an attack against North Korea and “big, big trouble” for its leader Kim Jong Un.

Mr Trump’s comments, however, did not appear to be backed by significant military mobilisation on either side of the Pacific, and an important, quiet diplomatic channel remained open.

“If he utters one threat in the form of an overt threat – which by the way he has been uttering for years and his family has been uttering for years – or he does anything with respect to Guam or any place else that’s an American territory or an American ally, he will truly regret it and he will regret it fast,” Mr Trump told reporters at his New Jersey golf resort.

Asked if the US was going to war, he said cryptically: “I think you know the answer to that.”

The compounding threats came in a week in which long-standing tensions between the countries risked abruptly boiling over.

New United Nations sanctions condemning the North’s rapidly developing nuclear programme drew fresh anger and threats from Pyongyang.

Mr Trump responded by vowing to rain down “fire and fury” if challenged and the North then threatened to lob missiles near Guam, a tiny US territory some 2,000 miles from Pyongyang.

Meanwhile talks between senior US and North Korean diplomats continue through a back channel previously used to negotiate the return of Americans held in North Korea.

The talks, which have expanded to address the deterioration of the relationship, have not quelled tensions, but could be a foundation for a more diplomacy, according to US officials and others briefed on the process.

A man in Seoul watches a TV screen showing US President Donald Trump and North Korean leader Kim Jong Un. (AP)
A man in Seoul watches a TV screen showing US President Donald Trump and North Korean leader Kim Jong Un. (AP)

On Friday Mr Trump sought to project military strength, only dialling back slightly throughout the day.

He began with a morning tweet: “Military solutions are now fully in place, locked and loaded, should North Korea act unwisely. Hopefully Kim Jong Un will find another path!”

He later retweeted a posting from US Pacific Command that showed B-1B Lancer bomber planes on Guam that “stand ready to fulfil USFK’s #FightTonight mission if called upon to do so”.

Such declarations, however, do not indicate a new, more aggressive posture.

“Fight tonight” has long been the motto of US forces in South Korea to show they are always ready for combat on the Korean Peninsula.