US President-elect Donald Trump's "clear warning" to North Korea shows he is aware of the urgency of the threat posed by its nuclear programme and will not waver from a policy of sanctions against the isolated country, South Korea has said.

North Korean leader Kim Jong Un said on Sunday that his nuclear-capable country was close to test-launching an intercontinental ballistic missile, raising the prospect of putting parts of the United States in range.

Taking to Twitter, Mr Trump dismissed the claim, saying "It won't happen."

South Korea's foreign ministry said Mr Trump's comment, his first mention of the North Korean nuclear issue since the US election in November, could be interpreted as a "clear warning" to the North.

"Because of our active outreach, President-elect Trump and US officials are clearly aware of the gravity and urgency of the North Korean nuclear threat," ministry spokesman Cho June-hyuck told a briefing.

"They are maintaining an unwavering stance on the need for sanctions on North Korea and for close cooperation between South Korea and the US".

Mr Trump has not outlined a policy on North Korea, but during the US election campaign indicated he would be willing to talk its leader, Mr Kim, given the opportunity.

He has also been critical of China over the issue. Yesterday, Mr Trump said China had benefited from its economic ties with the US but would not use its influence to help control North Korea.

Responding to the comment, Chinese Foreign Ministry spokesman Geng Shuang said China had been pushing for the denuclearisation of the Korean peninsula.

"China's efforts in this regard are perfectly obvious," Mr Geng told a news briefing. "

As a permanent member of the UN Security Council we have proactively participated in relevant discussions on the North Korean nuclear issue and have jointly passed several resolutions with other parties.

"This shows China's responsible attitude," Mr Shuang added.

For years the US has dismissed North Korean calls for talks, insisting it must disarm first.

Instead, the US and ally South Korea have responded to two North Korean nuclear tests and various missile tests last year with ever-more severe sanctions.

The UN Security Council imposed new sanctions on North Korea at the end of November after the country carried out its fifth and largest nuclear test so far in September.

A North Korean ICBM, once fully developed, could threaten the continental US, which is about 9,000 km from the North.

ICBMs have a minimum range of about 5,500 km, but some are designed to travel 10,000 km or further.

North Korea worked last year on developing components for an ICBM, making the claim that it was close to a test-launch plausible, international weapons experts said yesterday.