American nuclear arsenal is now far stronger and "more powerful than ever", US President Donald Trump said in an apparent warning to North Korea over its repeated missile tests.

Trump's tweets come a day after he told reporters that North Korea's threats against the US, if they continue, will be met with "fire and fury" like the world has never seen before.

"My first order as President was to renovate and modernise our nuclear arsenal. It is now far stronger and more powerful than ever before," Trump said in the first of his two tweets.

"Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!" Trump said in another tweet.

A day earlier, Trump warned North Korea of consequences of threatening the US.

"North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen. He has been very threatening, beyond a normal statement," Trump told reporters at his vacation home in New Jersey.

He was responding to reporters' questions on the North Korea's nuclear capabilities.

"As said, they will be met with fire, fury, and, frankly, power the likes of which this world has never seen before," he said.

Trump's remarks came amidst reports that North Korea is coming close to obtaining an atomic weapon that can strike the US.

Media reports said North Korea has successfully miniaturised a nuclear warhead that can fit inside its missiles.

House Democratic leader Nancy Pelosi slammed Trump for making such a remark.

"Since President Trump took office, North Korea has accelerated its nuclear programme to unprecedented levels. The president's most recent comments are recklessly belligerent and demonstrate a grave lack of appreciation for the severity of the North Korean nuclear situation," she said.

"The US must respond to the North Korean nuclear threat by working with other nations including Russia and China to enforce the new United Nations sanctions and must join with our allies to advance a diplomatic solution," she said.

Senator Dianne Feinstein said isolating the North Koreans has not halted their pursuit of nuclear weapons.

"And President Trump is not helping the situation with his bombastic comments. There is no question that North Korea is seeking to add a nuclear warhead to an ICBM capable of reaching the United States," she argued.

"What this tells me is that our policy of isolating North Korea has not worked. The United States must quickly engage North Korea in a high-level dialogue without any preconditions. Hopefully, Secretary Tillerson is already discussing the possibility of reopening talks with our Asian partners during his current trip. In my view, diplomacy is the only sound path forward," Feinstein said.

According to Arms Control Association, a peaceful resolution to the escalating crisis is more difficult than ever to achieve.

"Trump's attempt to play the role of nuclear 'madman' is as dangerous, foolish, and counterproductive as North Korea s frequent hyperbolic threats against the United States," it said in a statement.

"Trump's latest statement is a blatant threat of nuclear force that will not compel Kim to shift course. In fact, repeated threats of US military force only give credibility to the North Korean propaganda line that nuclear weapons are necessary to deter US aggression, and it may lead Kim to try to accelerate his nuclear programme," Arms Control Association said.