"We're in very close touch with our close allies and partners starting with the Republic of Korea, also with Japan and others, to be able to respond quickly should the North Koreans proceed with such a test," Blinken told reporters.

"And we are prepared to make both short- and longer-term adjustments to our military posture as appropriate," he said.

But he reiterated the United States has "no hostile intent toward the DPRK," the formal name of the North, the Democratic People's Republic of Korea.

"So we'll continue to reach out to the DPRK. We're committed to pursuing a diplomatic approach," Blinken said.

The top US diplomat was speaking after talks in Washington with his counterpart from South Korea, formally known as the Republic of Korea, who like Blinken said his government was ready to negotiate with Pyongyang "without preconditions."

"I think that North Korea has now finished the preparation for another nuclear test and I think only political decision has to be made," Foreign Minister Park Jin said.

"If North Korea ventures into another nuclear test I think that it will only strengthen our deterrence and also international sanctions," he said.

The United States attempted to toughen sanctions on North Korea after it tested an intercontinental ballistic in defiance of UN resolutions but China and Russia vetoed the bid at the Security Council.