A woman watches a TV screen at a train station in Seoul, June 5, showing a news program reporting about a North Korean missile launch with file footage of North Korean leader Kim Jong-un. North Korea test-fired a salvo of multiple short-range ballistic missiles toward the sea Sunday, South Korea's military said, extending a provocative streak in weapons demonstrations this year that U.S. and South Korean officials say may culminate with a nuclear weapon test. AP-Yonhap

The United States is closely monitoring the possibility of North Korea conducting its seventh nuclear weapon test, which if conducted, will prompt a robust response from the U.S., National Security Advisor Jake Sullivan said Wednesday.

However, he said the U.S. remains open to dialogue and will continue to do so.

"We are also watching very closely the continuing possibility of a nuclear test, to which we would also have a robust response," Sullivan told reporters aboard Air Force One while en route to California with President Joe Biden for the upcoming Summit of Americas there.

His remarks come one day after U.S. Special Representative for North Korea Sung Kim said the U.S. and its allies will have a "swift response" against any North Korean nuclear test.