National Security Advisor Jake Sullivan is seen answering questions during a press briefing at the White House in Washington on Thursday in this image captured from the website of the White House. (Yonhap)

WASHINGTON -- The United States continues to remain concerned about a potential North Korean nuclear test, National Security Advisor Jake Sullivan said Thursday, noting that Pyongyang may still decide to conduct a test.

Sullivan also expressed concerns over additional North Korean missile tests.

"We have been very transparent about out concern with North Korea that the DPRK would conduct its seventh nuclear test at some point in this broader timeframe, and by broader I mean we have been talking about this now for a couple of months," he told a press briefing when asked if the North may be expected to conduct a nuclear test during President Joe Biden's upcoming trip to North Africa and Asia.

"That window still remains open. Our concern still remains real," added Sullivan.

DPRK stands for the Democratic People's Republic of Korea, the North's official name.

Officials here and in Seoul have said the North may have completed all preparations for what will be its seventh nuclear test and that it may only be waiting for a decision from leader Kim Jong-un.

Sullivan also highlighted the possibility of North Korea conducting additional "long-range" missile tests.

"We are also concerned about further potential long range missile tests in addition to the possibility of a nuclear test. And so we'll be watching carefully for both of those," Sullivan said.

North Korea fired an intercontinental ballistic missile early last week, marking its seventh ICBM test launch of the year.

Pyongyang has fired some 60 ballistic missiles this year.

Seoul earlier said President Yoon Suk-yeol will hold a bilateral summit with President Biden when they attend regional summits in Cambodia and Indonesia.

Sullivan said the Biden-Yoon summit, along with a trilateral summit involving the two leaders and Japanese prime minister, will be held in Cambodia, where the leaders will attend the East Asia Summit that also involves the 10-member Association of Southeast Asian Nations.

"We are in very tight coordination with both the ROK and Japan, and the trilateral meeting that the three leaders will be able to have on the margins of the East Asia Summit, I think, will be a significant milestone in the trilateral relationship and will help only strengthen coordination on DPRK," he said, referring to South Korea by its official name, the Republic of Korea.