North Korea has ignored numerous U.S. dialogue initiatives as well as offers to help combat the Covid-19 outbreak, the US Special Representative for North Korea, Sung Kim said.

He said that Pyongyang has not responded to months of public and private messages aimed at resolving tensions.

President Joe Biden and Secretary of State Antony Blinken have repeatedly stated publicly that Washington seeks diplomatic talks with the Democratic People's Republic of Korea "without preconditions," Kim said.

"We have also reached out to pass this message through private channels as well. This includes high-level personal messages from senior US officials to senior DPRK officials," he told reporters in a briefing.

"Over the past year, we have sent such messages in multiple ways, through third parties, directly in writing," he added, noting that included North Korea neighbor China.

Such messages included specific offers of humanitarian cooperation and assistance in response to the recent Covid-19 outbreak in North Korea.

U.S. officials have shown growing concern over Pyongyang's intensified nuclear activities. According to Kim, Washington believes Pyongyang will soon conduct its seventh nuclear weapons test after a break of nearly five years.

U.S. Undersecretary of State Wendy Sherman, speaking in Seoul, warned that the response would be "swift and forceful."

"Any nuclear test would be a complete violation of UN Security Council resolutions," she said.