North Korea warned Thursday it would take "merciless self-defensive" measures should the United States enforce a naval blockade, which Pyongyang sees as "an act of war," the isolated nation's state media said.

Citing a foreign ministry spokesperson, the North's KCNA news agency said a naval blockade would be a "wanton violation" of the country's sovereignty and dignity, adding that North Korea was a responsible nuclear power.

U.S. President Donald Trump was taking an "extremely dangerous and big step toward the nuclear war" by seeking such a blockade, it added. It was not immediately clear what U.S. proposal the agency was referring to.

"Should the United States and its followers try to enforce the naval blockade against our country, we will see it as an act of war and respond with merciless self-defensive counter-measures, as we have warned repeatedly," the agency said.

U.S. strike would be 'catastrophic'

The Chinese president said Thursday that war must not be allowed to break out on the Korean peninsula.

President Xi Jinping made the comments to visiting South Korean President Moon Jae-in just days after U.S. Secretary of State Rex Tillerson offered to begin direct talks with North Korea without pre-conditions.

But the White House said on Wednesday that no negotiations could be held until North Korea improved its behaviour.

The White House has declined to say whether President Donald Trump, who has taken a tougher rhetorical line toward North Korea, approved Tillerson's overture.

Russian President Vladimir Putin, speaking in his annual end-of-year address Thursday, said Tillerson's offer of direct contacts with North Korea was "a very good signal," while warning that a U.S. strike on the North would have "catastrophic" consequences.

British Foreign Secretary Boris Johnson said Thursday that the best way forward on resolving the North Korean crisis is to intensify economic pressure on the secretive state.

Johnson, speaking alongside Japanese Foreign Minister Taro Kono in London, said military options in North Korea did not look attractive.

Kono said North Korea was the most urgent and important threat faced by international community.

UN resolutions 'must be implemented'

North Korea tested its most advanced intercontinental ballistic missile on Nov. 29, which it said could put all of the U.S. within range, in defiance of international pressure and United Nations sanctions.

"Security Council resolutions must be fully implemented, first of all by North Korea, but by all other countries whose

role is crucial," UN Secretary General Antonio Guterres, speaking to reporters in Tokyo after meeting Japanese Prime Minister Shinzo Abe.

He said he expected a meeting of the UN Security Council on Friday would deliver a strong expression of unity and the need for diplomacy to resolve the issue.

"The worst possible thing that could happen is for us all to sleepwalk into a war."

North Korea justifies its weapons programs as necessary defence against U.S. plans to invade. The United States, which has 28,500 troops in South Korea, a legacy of the 1950-53 Korean War, denies any such intention.