Bolton made the remarks on Fox Business following a series of missile tests by North Korea in the past week. South Korea described the projectiles as short-range ballistic missiles, but the North said the second test on Wednesday involved a new rocket launch system.

The launches came despite Washington's efforts to restart working-level talks with North Korea in line with an agreement reached by US President Donald Trump and North Korean leader Kim Jong-un when they met at the inter-Korean border on June 30.

"We've been waiting to hear since June the 30th," Bolton said.

"We're ready for working-level negotiations. President's ready when the time is right for another summit. Let's hear from North Korea."

Bolton, who is known for his hawkish views on the North, has been seen supporting the president's commitment to continuing diplomatic engagement with the regime despite little indication it intends to give up any of its nuclear weapons and ballistic missile programs.

On Wednesday he told Fox Business that the recent launches did not represent a violation of Kim's promise to Trump not to conduct long-range missile tests.

"I think (the promise) is part of the basis on which the president still has optimism we could negotiate with Kim Jong-un," Bolton elaborated in the interview Thursday. "The South Koreans and Japanese are concerned nonetheless because they are within range, we think, of this particular missile."

South Korea's military said the two short-range ballistic missiles on Wednesday were estimated to have flown about 250 kilometers at an approximate altitude of 30 km.

Bolton emphasized that Trump is committed to helping the North develop its economy if it abandons its nuclear program.

"He's held the door open for Kim Jong-un," the adviser said.

"He's shown him the brighter and different future North Korea would have if they would make the strategic decision to give up nuclear weapons."

Asked what Trump considers to be the biggest threat to the US, Bolton cited the proliferation of nuclear weapons and other weapons of mass destruction, saying that is why the president focuses on North Korea and Iran. (Yonhap)