WASHINGTON -- US President Donald Trump wants a diplomatic solution to the nuclear standoff with North Korea and will continue to work on it "until the first bomb drops," his top diplomat said Sunday.

Secretary of State Rex Tillerson made the remark in an interview on CNN as tensions have heightened over North Korea's nuclear and ballistic missile programs.

"He has made it clear to me to continue my diplomatic efforts," Tillerson said. "Those diplomatic efforts will continue until the first bomb drops."

Trump appeared to undercut his top diplomat on Twitter early this month, saying Tillerson is "wasting his time trying to negotiate with Little Rocket Man," a reference to North Korean leader Kim Jong-un.

"Save your energy Rex, we'll do what has to be done!" he added.

Tillerson said the president was trying to "motivate action" from a number of people, especially the North Korean regime.

"I think he does want to be clear with Kim Jong-un -- that regime in North Korea -- that he has military preparations ready to go and he has those military options on the table," he said. "And we have spent substantial time actually perfecting those.

"But be clear -- the president has also made clear to me he wants this solved diplomatically. He's not seeking to go to war," Tillerson said.

North Korea conducted a series of ballistic missile and nuclear tests in recent months, demonstrating a growing capability to strike the US mainland with a nuclear weapon.

Trump and Kim have engaged in a war of words with threats to take military action against each other. (Yonhap)