TOKYO — As Hillary Rodham Clinton arrived in Japan on Monday, her first foreign visit as secretary of state, North Korea threatened to test what its neighbors believe is a ballistic missile capable of reaching the United States.

Speaking briefly during an arrival ceremony at the airport, Mrs. Clinton did not directly address the reports from North Korea, but on her flight she continued to employ a tone that was notably softer than previous American pronouncements, echoing remarks she made in New York last week. She also repeated her offer to normalize ties with North Korea and help rebuild its economy if it abandoned its nuclear weapons program.

But in what appeared to be a pointed challenge to her mission on this trip to Asia and a test for the Obama administration, North Korea issued an oblique statement responding to recent news reports that it had been preparing to test-launch a Taepodong-2 missile from a base on its east coast.

“One will come to know later what will be launched,” the North’s state-run news agency, KCNA, said Monday.

In Seoul, the South Korean defense minister, Lee Sang-hee, said North Korea had been preparing to test a Taepodong-2 missile since January. In recent weeks, South Korean news media have reported that North Korean engineers were assembling a 105-foot Taepodong-2 missile.

Analysts and government officials in the region have feared that North Korea may launch a long-range missile to help make its nuclear program a top foreign policy issue for President Obama. With its economy in shambles and isolated from most of the world, North Korea has often used military threats to extract economic aid and diplomatic benefits from other countries.

As she landed in Tokyo, Mrs. Clinton appeared to be sticking with her previously scheduled agenda for a tour of four Asian countries this week, intended to build solidarity on issues like the global economic crisis and climate change.