Kerry said nations must agree on a "swift, clear, strong and credible response" to Pyongyang's third nuclear test and the authoritarian regime's "continued flaunting of its obligations."
In defiance of U.N. Security Council resolutions, the latest issued last month, North Korea on Tuesday detonated a nuclear device at a remote underground site. It is seen as a key step toward its goal of building a bomb small enough to be fitted on a missile capable of striking the United States.
Iran, like North Korea, is also under stiff sanctions, and negotiations with the West over its nuclear program have similarly stalled.
Iran maintains the program is peaceful, for generating energy and for medical research, not for weapons. It said Wednesday that it has begun installing a new generation of centrifuges that will allow it to vastly increase its pace of uranium enrichment in defiance of U.N. calls to halt such activities.
There has been speculation that North Korea and Iran could be cooperating on missile and nuclear development. Kerry did not draw such a connection but did say the cases were linked because they both concerned nonproliferation.
"It's important for the world to have credibility with respect to our nonproliferation efforts," Kerry told reporters after meeting Jordanian Foreign Minister Nasser Judeh at the State Department.
"Just as it's impermissible for North Korea to pursue this kind of reckless effort, so we have said it's impermissible with respect to Iran. What our response is with respect to this will have an impact on all other nonproliferation efforts."
The U.N. Security Council has issued three separate resolutions on North Korea, in response to its nuclear and missile tests since 2006. The latest resolution, which tightened sanctions, followed a December satellite launch that the U.S. says could serve to develop the North's ballistic missile capability.
The resolution warned of "significant action" if Pyongyang conducted another rocket launch or a nuclear test.
"If you are going to say things, they have to mean something. And to mean something you have to be prepared to follow up, and that's exactly what we are prepared to do," Kerry said.
European Union foreign policy chief Catherine Ashton, speaking Wednesday at the Security Council, called North Korea's nuclear test "a further blatant challenge to the global non-proliferation regime."
She said it was vital for the international community to stand united and demonstrate "that there are consequences of continued violations."
In Washington, President Barack Obama and Japanese Prime Minister Shinzo Abe consulted on a response and pledged to work together to seek action by the U.N. Security Council, the White House said.
Defense Secretary Leon Panetta said North Korea's actions represent a clear threat to the United States and undermines regional security. He vowed that the U.S. will take whatever steps necessary to meet its commitment to South Korea, including continued military exercises and increases in missile defense in the region.
Panetta said U.S. technical experts are still assessing available data to learn more about the underground explosion. That difficult task could determine whether the device was made with plutonium, of which North Korea has only limited supplies, or uranium, which can be enriched to weapons-grade in more easily concealed facilities.
"It should be of great concern to the international community that they are continuing to develop their capabilities to threaten the security, not only of South Korea, but of the rest of the world," Panetta told a Pentagon news conference. "And for that reason, I think that we have to take steps to make very clear to them that that kind of behavior is unacceptable."
The U.S. is fully prepared to deal with any contingency involving North Korea, Panetta said, and will continue to deploy forces to that region.
In an emergency session Tuesday, the Security Council unanimously said the nuclear test poses "a clear threat to international peace and security" and pledged further action. It remains to be seen, however, whether China, the North's ally, will sign on to any new, binding global sanctions.
Associated Press writers Bradley Klapper and Robert Burns contributed to this report.