U.S. Special Representative for North Korea, Sung Kim / AP-Yonhap

North Korea appears to have completed preparations for what will be its seventh nuclear test that could take place at any time, Special U.S. Representative for North Korea Sung Kim said Tuesday.

Kim said a North Korean nuclear test will be met by a "swift" response from the U.S. and its allies, but that Washington will continue to remain open to dialogue with no "end date."

"On timing, I don't have anything more," Kim said when asked if the U.S. agreed with Seoul's earlier assessment that North Korea may conduct a nuclear test as early as this week.

"They have obviously done the preparations in Punggye-ri and I understand they could test any time," he added, referring to North Korea's key nuclear test site.

North Korea conducted its sixth and last nuclear test in September 2017.