The White House has warned that the U.S. will respond strongly if North Korea conducts its seventh nuclear test amid speculation that the North is ready for a test at its Punggye-ri nuclear site.

U.S. National Security Advisor Jake Sullivan told reporters on Wednesday that the U.S. is very closely watching the continuing possibility of a nuclear test by the North, to which it would have a "robust response."

He added that the U.S. continues to monitor and respond to the missile tests that the North has been conducting.

Sullivan said the North's actions have only cemented the strength of the U.S. alliances with South Korea and Japan, citing the U.S. joint exercises with the allies in response to the North Korean missile launches.

The top security adviser, however, stressed that the U.S. remains open and willing to engage in diplomacy if North Korea is prepared to come to the table. He also reaffirmed the U.S.’ resolute commitment to defending its allies South Korea and Japan.