A top US lawmaker warned North Korea Thursday that any pre-emptive nuclear strike against America would be suicide, as senators pushed for a review of policy towards the isolated state.

"I do not think the regime in Pyongyang wants to commit suicide, but that, as they must surely know, would be the result of any attack on the United States," Senator Bob Menendez told the Senate Foreign Relations Committee.

He was speaking as the United Nations adopted new sanctions on North Korea for last month's nuclear test, and as Pyongyang said a new war was "unavoidable" on the peninsula because of South Korean-US military exercises.

The North's military "will exercise the right to a pre-emptive nuclear attack to destroy the strongholds of the aggressors," Pyongyang's foreign ministry said.

Menendez said the threat was "absurd," but he warned: "There should be no doubt about our determination, willingness, and capability to neutralize and counter any threat that North Korea may present."

North Korea has accumulated some 20 to 40 kilos of plutonium, "enough for perhaps six to eight nuclear weapons," Menendez said.

It is also seeking the capability to fit a nuclear warhead to an intercontinental ballistic missile, with some estimates mentioned in the committee that it may be only months or a year away from achieving that.

"In time, if its present course remains unaltered, North Korea will pose a direct threat to the United States," Menendez warned at a hearing to consider US policy towards Pyongyang.

Ranking Republican member Senator Bob Corker accused US administrations of failing in their policies towards the communist-run state, and of not dissuading its closest ally, China, to stop supporting Pyongyang.

North Korea was "a complex policy conundrum" to which "there is no silver-bullet solution," Corker said.

"Yet after nearly 20 years of unsuccessful policies by successive administrations, it seems logical to me that we ought to undertake a comprehensive review of our North Korean strategy, including harnessing new tools to try to crack the North Korean policy nut."

The US special representative for North Korea, Glyn Davies, admitted progress would not "achieved easily" but he challenged Corker's view.

"We cannot and should not dignify or, worse, feed the North Korean narrative that US actions determine DPRK behavior," he said in his written testimony.

"North Korea makes its own choices, selects its own timing, and is alone responsible for its actions."

But he also laid out US principles for dealing with North Korea, also known as the DPRK, as it veers into new waters under fledgling leader Kim Jung-Un.

"First and foremost, the United States will not accept North Korea as a nuclear-armed state," he said.

"We will not reward the DPRK for the absence of bad behavior. We will not compensate the DPRK merely for returning to dialogue. We will not tolerate North Korea provoking its neighbors."

Davies also stressed that ties would not improve until Pyongyang's relations with South Korea are boosted and the human rights situation in the isolated state is addressed.

A network of political prison camps across the country is reported to hold between 100,000 to 200,000 people including children, he added.

"While denuclearization remains an essential focus of US policy, so, too, does the welfare of North Korea's nearly 25 million people, the vast majority of whom bear the brunt of their government's decision to perpetuate an unsustainable, self-impoverishing military-first policy," he said.