"If Kim Jong Un decides to launch a missile, whether it is across the Sea of Japan or some other direction, he will be choosing willfully to ignore the entire international community," Kerry said at a news conference in Seoul after meeting with South Korea's President Park Geun-hye and Foreign Minister Yun Byung-se.
Kerry arrived in the South Korean capital Friday on a trip designed to reassure Asian allies of U.S. commitments to defend them. He will also visit China and Japan, The New York Times reported.
The Asia visit follows Kerry's trips to Turkey and Israel and his participation in the Group of Eight industrialized nations' meeting in London.
Kerry's Asia visit comes amid growing concerns in Seoul and Tokyo that North Korea is preparing to test-launch a ballistic missile, raising tensions in the region. Japan's Defense Ministry has already deployed its anti-missile units around Tokyo.
North Korea, which has threatened to launch pre-emptive nuclear strikes that could hit U.S. bases in Guam, could fire a medium-range Musudan missile during its test.
North Korea was expected to top the agenda in the talks between Kerry and South Korean officials, Yonhap News reported.
The Times, quoting a U.S. State Department official traveling with Kerry, said during his China visit the secretary of state would urge his hosts to persuade North Korea to abandon its nuclear weapons program.
Among its latest threats, North Korea has said it would restart its idled nuclear reactor.
The U.S. official said Washington wants China to crack down on the money flow used by North Korea through its companies and banks to finance its nuclear weapons effort, the Times reported.
"We want to see them do what we do, what the Japanese do, what the South Koreans do, which is to stick to U.N. Security Council resolutions" and "stop those money trails," the official told the Times.
The United States also wants China to make it clear to the North about the need for denuclearization.