Speaking in Washington after his meeting with visiting South Korean Foreign Minister Yun Byung-se, Kerry assured the political partnership between the two countries is a "significant example" of their commitment to engagement in the Asia Pacific and that "our alliance" is the linchpin of security and stability in Northeast Asia.
"The United States and the Republic of Korea (official name of South Korea) stand very firmly united, without an inch of daylight between us, not a sliver of daylight, on the subject of opposition to North Korea's destabilizing nuclear and ballistic missile programs and proliferation activities. And the international community stands with us," Kerry said.
The secretary also said the United States supports South Korean President Park Geun-hye's "firm, principled approach" toward North Korea.
He urged North Korea to start fulfilling its international obligation and commitments relating to its denuclearization.
Yun's visit comes even as North Korea under its unpredictable Kim Jing Un continues with its purges and other efforts to consolidate his two-year-old leadership and to remove "factionalists lurking in the party."
The purges have included the execution of Jang Song Thaek, the 67-year-old uncle and mentor of the North Korean leader who had been the country's second-most powerful man prior to his death. North Korea remains under tough U.N. sanctions for its third nuclear test last year and its long-range missile in defiance of U.N. resolutions.
"We will not accept North Korea as a nuclear state nor as a nuclear-armed state, and nor will the international community abide by that," Kerry said.
He said the United States remains fully committed to the defense of South Korea, including "through extended deterrents and putting the full range of U.S. military capabilities in place," and that the two sides will continue to modernize "our capabilities so that we are prepared to face any threat."
Yun said he and Kerry agreed to enhance policy coordination "to tackle North Korean issues" and "seeking progress in the resolution of the North Korea nuclear problem."
"In the event of any North Korean provocation, South Korea and the United States will firmly respond based on our robust combined defense posture," Yun said.
He said South Korea welcomes the United States' new policy of rebalancing to Asia as a "positive contribution to the stability and prosperity in the region."