Kerry, speaking to reporters after meeting with South Korean Foreign Minister Yun Byung-se, answered questions about North Korea's latest decision to restart its nuclear facility.
"I'm not going to speculate on what the intent is or whether there's a strategy or not a strategy," Kerry said, a State Department transcript showed. "The bottom line is very simply that what (North Korean leader) Kim Jong Un has been choosing to do is provocative, it is dangerous, reckless, and the United States will not accept DPRK (Democratic People's Republic of Korea, the official name of North Korea) as a nuclear state.
"And I reiterate again the United States will do what is necessary to defend ourselves and defend our allies, Korea and Japan. We are fully prepared and capable of doing so, and I think the DPRK understands that."
Kerry said at the same time the developments in the region are not being taken lightly, "which is precisely why the President made the decision to redeploy missile defense with respect to the United States itself as well as to take other preparations in the region and to send a very clear signal to our allies and the North alike that the United States will defend our allies and that we will not be subject to irrational or reckless provocation."
The simple way for North Korea is to rejoin the community of nations and make it clear they want to pursue a peaceful path, he added.
Asked if he believed the North would restart its Yongbyon nuclear facility as announced, Kerry said such a step would be in "direct violation" of its international obligations.
"So we'll have to wait and see what happens with respect to that, but it is a direct violation of their international obligations and would be a very serious step," he said.