North Korea has vowed to rapidly strengthen its nuclear-strike capability as long as it faces a "hostile" US policy.

At the United Nations, North Korea's deputy ambassador, Kim In Ryong, said Pyongyang would never abandon its "nuclear deterrence for self-defence and pre-emptive strike capability" even if the US ratchets up sanctions and pressure "to the utmost".

North Korea tested a longer-range missile last weekend, which experts say was a significant advance for a weapons programme that aims at having a nuclear-tipped missile that can strike America.

The test triggered a new US-backed push for a fresh round of UN sanctions against the North.

Speaking to reporters, Kim hailed the test launch and said that if the Trump administration wants peace on the divided Korean Peninsula, it should replace the armistice that ended the 1950-53 Korean War with a peace accord and halt its anti-North Korea policy.

All 15 members of the UN Security Council, the world organisation's most powerful body, this week called the launch a violation of existing sanctions and vowed to take new measures, including additional sanctions.

Kim accused the council of playing "to the tune of the US again" and protested against the Trump administration's demand for countries to choose allegiance between the US and the Democratic People's Republic of Korea, his country's official name.

Asked about Beijing and Moscow's support for the six previous rounds of UN sanctions, Kim said both countries are "close neighbours" who "understand our nuclear projection occurred through the US continued nuclear threat and its hostile policy" toward North Korea.

If the United States "persists in anti-DPRK sanctions without understanding its rival, the [Trump] administration will have to take full responsibility for the ensuing catastrophic consequences," he warned.

"The US should mind that the DPRK nuclear striking capability will be strengthened and developed at a rapidly high speed as long as the US insists [on] its anti-DPRK policy, nasty nuclear threats and blackmails, sanction and pressure," Kim said.

At the Pentagon, Defense Secretary Jim Mattis stressed the need for a peaceful resolution by working through the UN with countries including China, the North's traditional ally and benefactor.



"If this goes to a military solution it is going to be tragic on an unbelievable scale, and so our effort is to work with the UN, work with China, work with Japan, work with South Korea to try to find a way out of this situation," Mattis said at a news conference.