"We are preparing for all contingencies, including the possibility that such a provocation would occur while we are in Korea or in Japan," Sullivan told a White House briefing.
Sullivan said the United States was coordinating closely with South Korea and Japan on the issue and had also discussed North Korea with his Chinese counterpart Yang Jiechi about North Korea in a phone call on Wednesday.
"We've indicated in quite clear terms that our intelligence does reflect a genuine possibility that there will be either a further missile test, including long-range missile test, or a nuclear test, or frankly both, in the days leading into, on, or after the president's trip to the region," Sullivan said.
He said the United States was prepared to make both short and longer term adjustments to its military posture as necessary "to ensure that we are providing both defense and deterrence to our allies in the region and that we're responding to any North Korean provocation."
White House spokesperson Karine Jean-Pierre told the same briefing Biden would not visit the Demilitarized Zone (DMZ) that divides North and South Korea during his visit to South Korea, which begins on Friday.
The White House said last week Biden was considering such a trip.
"He will he will not visit the DMZ ... not on this trip," Jean-Pierre said.
(Reporting by David Brunnstrom, Alex Alper, Caitlin Webber and Jarrett Renshaw; Editing by Sandra Maler; Editing by Chris Reese)