South Korea's Foreign Ministry said Tuesday that Donald Trump's comment - his first mention of the North Korean nuclear issue since he was elected US president in November - could be interpreted as a "clear warning" to the North.

Trump's tweet Monday came in response to comments from North Korea's Kim Jong Un a day earlier, in which he said his nuclear-capable country was close to test-launching an intercontinental ballistic missile (ICBM), alluding the prospect of putting parts of the US in range.

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

"Because of our active outreach, President-elect Trump and U.S. officials are clearly aware of the gravity and urgency of the North Korean nuclear threat," Foreign Ministry spokesman Cho June-hyuck said at a briefing on Tuesday.

"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 open to talks

Trump is yet to outline a policy on North Korea. During the US election campaign, however, he indicated he would be willing to talk its leader, Kim, given the opportunity. The US has for years dismissed North Korean calls for talks, insisting it must first disarm. 

Speaking on ABC's "Good Morning America" program on Tuesday, Trump spokeswoman Kellyanne Conway said the president-elect would not publicly state how he might respond to North Korea's nuclear weapons and missile programmes "before he's inaugurated" on January 20.

"We do know that there are sanctions that are possible," Conway said. "They haven't always worked. I think that China would have to have a significant role here as well."

Criticism of China

In a tweet from Trump criticized China, North Korea's most important ally, for not doing more to discourage its nuclear weapons program.

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

While Beijing has publicly reprimanded Pyongyang after nuclear tests and has agreed to rounds of UN sanctions against the North, critics say China hasn't done enough to tighten economic pressure. 

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

Chinese foreign ministry spokesman Geng Shuang responded to Trump's tweet, saying China's efforts and commitment to the dismantling of North Korea's nuclear program are "consistent and clear." 

"The effort China has made to this end is obvious to all," Geng said. "We hope all sides can refrain from speaking or doing anything that can aggravate the situation and work in concert to pull the issue back to dialogue and negotiation."

ksb/rc (AP, Reuters)