"We are doing all we can to protect the safety of our nation," chief Cabinet spokesman Yoshihide Suga said, though he and Ministry of Defense officials refused to confirm the reports about the naval alert, saying they do not want to "show their cards" to North Korea.

Japanese officials long have feared that North Korea not only has the means but several potential motives for launching an attack on Tokyo or major U.S. military installations on Japan’s main island.

“We will stand with South Korea and Japan against these threats. And we will defend ourselves," he said.

Kerry also weighed in on an intelligence report that rocked Washington on Thursday and suggested that North Korea now had the knowhow to arm a ballistic missile with a nuclear warhead -- even if the weapons would lack reliability. Citing the Pentagon's assessment, Kerry rejected the finding and said that Pyongyang still hadn't developed or fully tested the nuclear capacities needed for such a step.

Speaking beside Kerry, South Korea Foreign Minister Yun Byung-se called for more United Nations action against Pyongyang if it commits another provocation.

He refused to comment specifically on the U.S. intelligence report, saying only that the North has "high nuclear and missile capabilities" but that it is still some time away from a nuclear bomb that is "small, light and diversified."

Kerry offered strong words of solidarity for South Korea, and praised South Korea President Park Geun-hye’s "bright vision" of a prosperous and reunified Korean Peninsula without nuclear weapons. By contrast, he said North Korea's Kim,  has a choice to make between provocation and returning to talks to de-escalate tension and lead to the end of its nuclear program

Both Yun and Kerry kept the door open for future negotiations with Pyongyang.