Washington (Reuters): 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 program and will not waver from a policy of sanctions against the isolated country, South Korea said on Tuesday.

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

Trump dismissed the claim, saying on Twitter: "It won't happen."

South Korea 's foreign ministry said 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".

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

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