North Korea said on Monday that it was not frightened by US threats of possible pre-emptive military action to halt its nuclear and missile build-up.

A spokesman for North Korea's Foreign Ministry slammed US Secretary of State Rex Tillerson's recent talk of tougher sanctions, more pressure, and possible military action, and said the North would not be deterred in its nuclear programme.

"The nuclear force of [North Korea] is the treasured sword of justice and the most reliable war deterrence to defend the socialist motherland and the life of its people," the official Korean Central News Agency quoted the spokesman as saying.

Trump says Kim Jong-un 'acting very, very badly'

Tillerson recently visited Japan, South Korea and China on trip that focused on North Korea's nuclear programme. On Friday, he signalled a tougher strategy that left open the possibility of pre-emptive military action.

"Let me be very clear: The policy of strategic patience has ended," he said after visiting the heavily militarised border between the rival Koreas.

"We are exploring a new range of diplomatic, security and economic measures. All options are on the table."

A day earlier, in Japan, Tillerson had described the past 20 years of US policy towards North Korea as a failure and vowed a comprehensive policy review under US President Donald Trump.

KCNA quoted the unidentified foreign ministry spokesman as saying the US should accept that North Korea is a nuclear-capable nation that "has the will and capability to fully respond to any war the US would like to ignite".

"If the businessmen-turned US authorities thought that they would frighten [North Korea], they would soon know that their method would not work," he said.

On Saturday, North Korea conducted a ground test of a new type of high-thrust rocket engine that leader Kim Jong-un called a revolutionary breakthrough for the country's space programme.

North Korea tests new high-thrust rocket engine

North Korea has accelerated its weapons development, violating a number of UN Security Council resolutions without being deterred by sanctions. It conducted two nuclear test explosions and 24 ballistic missile tests last year. Experts say it could have a nuclear-tipped missile capable of reaching the US mainland within a few years.

Trump told reporters on Sunday he held meetings on North Korea over the weekend and said Kim was "acting very, very badly".

China said on Monday that the situation with North Korea was at a new crossroads with two scenarios - a deterioration to war or a diplomatic solution.

"Any chance for dialogue must be seized, as long as there's hope," Foreign Minister Wang Yi said in Beijing.