US President Donald Trump has said that perhaps his “fire and fury” warning to North Korea “wasn’t tough enough”.

Mr Trump said North Korea had “better get their act together or they are going to be in trouble like few nations have ever been in trouble”.

The US president was addressing reporters during his holiday at his New Jersey golf club before a security briefing with top advisers.

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It is the latest warning since he said earlier this week that North Korea faces “retaliation with fire and fury unlike any the world has seen before”.

North Korea has said it may attack Guam in retaliation.

Referring to his comments earlier in the week Mr Trump said: “Maybe that statement wasn’t tough enough.”

Donald Trump and Mike PencePresident Donald Trump, accompanied by vice-president Mike Pence, speaks to reporters before a security briefing at Trump National Golf Club (Evan Vucci/AP)

He said North Korea had been “getting away with a tragedy that can’t be allowed”.

But he declined to say whether the US was considering a pre-emptive military strike, arguing that his administration never discusses such deliberations publicly.

Mr Trump said it was time that somebody stood up to the pariah nation.

Flanked by US vice president Mike Pence, he said: “It may very well be tougher than I said.”

Mr Trump said the US “of course” would always consider negotiations with North Korea, but added that negotiations with the North have failed for the last 25 years. He accused his predecessors of failing to effectively address the North Korea problem.

Alluding to the threats against Guam, Mr Trump said if North Korea took any steps to even think about an attack, it would have reason to be nervous.

“Things will happen to them like they never thought possible, OK?” Mr Trump said. Of North Korean leader Kim Jong Un, Mr Trump added: “He’s been pushing the world around for a long time.”

Hwasong-12 missile: projected trajectoryHwasong-12 missile: projected trajectory (PA graphic)

Mr Trump also took the opportunity to declare the opioid crisis in the US as a “national emergency”.

He told reporters the drug crisis afflicting the nation is a “serious problem the likes of which we have never had” and said he is drawing up documents “to so attest”.

A drug commission convened by Mr Trump recently called for a national emergency declaration to help deal with the opioid crisis.

Health and human services secretary Tom Price earlier this week seemed to suggest the president was leaning against the recommendation when he said the administration could deploy the necessary resources and attention without declaring a national emergency.

Still, Mr Price stressed that “all things” were “on the table for the president”.