Donald Trump has said North Korean leader Kim Jong Un "will regret it fast" if he continues his threats to US territories and allies.

In remarks to reporters, Mr Trump issued the threat directly at Kim, who is also known for his bellicose rhetoric, and all but drew a red line that would trigger swift US action.

"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 said.

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The words followed an early morning tweet in which Nr Trump declared the US military is "locked and loaded" if the isolated rogue nation acts "unwisely".

The threats came in a week in which the long-standing tensions between the US and the isolated nation seemed to abruptly boil over. North Korea threated to launch an attack on the US territory of Guam, while Mr Trump vowed to deliver "fire and fury" if threatened.

Tough talk aside, there was scant sign the US military was preparing for imminent action and an important, quiet diplomatic channel remained open.

The Associated Press reported on Friday that talks between North Koreans and a US official continue through a back channel previous used to negotiate the return of Americans held in North Korea.

The talks have expanded to address the deterioration of the relationship, according to US officials.

But Mr Trump on Friday sought to project the military strength.

He tweeted: "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 fulfill USFK's #FightTonight mission if called upon to do so".

Such declarations, however, are not necessarily indicators of 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.

US officials insist that there have been no new significant movement of troops, ships, aircraft or other assets to the region other than what has already been long planned for previously scheduled exercises.

American and South Korean officials said they would move forward later this month with the exercises, which North Korea claims are a rehearsal for war.

The days of war rhetoric have alarmed international leaders.

"I don't see a military solution and I don't think it's called for," said German Chancellor Angela Merkel.

"I think escalating the rhetoric is the wrong answer," she added.

Russia's foreign minister Sergey Lavrov estimated the risk of a military conflict between the US and North Korea as "very high," and said Moscow was deeply concerned.

"When you get close to the point of a fight, the one who is stronger and wiser should be the first to step back from the brink," Mr Lavrov said on Friday.