President-elect's tweet 'clear warning' to North Korea, says Seoul

"North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the US," Trump tweeted. "It won't happen!"

Trump's tweet came a day after North Korean leader Kim Jong-Un appeared to try to pressure the incoming president by announcing his country is in the "final stages" of developing an intercontinental ballistic missile (ICBM).

Kim also said his country had significantly bolstered its nuclear arsenal last year.

Washington has repeatedly vowed that it would never accept North Korea as a nuclear-armed nation, but Trump has not previously clearly stated his policy on the isolated Stalinist state.

"President-elect Trump's message is significant since it is his first mention of North Korea's nuclear programme and can be seen as a clear warning," South Korean foreign ministry spokesman Cho June-Hyuck told a briefing.

Cho said the incoming US administration was clearly aware of the "gravity and urgency" of Pyongyang's nuclear threat thanks to South Korea's "active outreach".

US policy on the North would remain largely unchanged, he said.

"They are maintaining an unwavering stance on the need for sanctions on North Korea," Cho said.

"China has been taking out massive amounts of money & wealth from the US in totally one-sided trade, but won't help with North Korea," he tweeted. "Nice!"

Following Trump's remarks, a Chinese foreign ministry spokesman again declared that Beijing would "stay committed to denuclearisation" on the peninsula.

"Efforts made by the Chinese side are obvious for all to see. We hope that relevant parties would refrain from words and deeds that will lead to the escalation of tension," Geng Shuang told reporters.

In a New Year's speech on Sunday, Kim did not make a specific reference to the incoming Trump administration. But he called on Washington to make a "resolute decision to withdraw its anachronistic hostile North Korea policy".

Analysts are divided over how close Pyongyang is to realising its full nuclear ambitions, especially as it has never successfully test-fired an ICBM.

However, it carried out two nuclear tests and numerous missile launches last year alone in pursuit of its oft-stated goal -- developing a weapons system capable of hitting the US mainland with a nuclear warhead.

Trump's tweet 'clear warning' to North Korea, says Seoul