SEOUL/TOKYO -- Closing out his two-day visit to South Korea, the first overseas tour by a senior Trump administration official, U.S. Defense Secretary James Mattis warned North Korea that any use of nuclear weapons would be met with an "overwhelming" response.
Heading into a meeting with his South Korean counterpart, Han Min-koo, on Friday, Mattis told reporters: "North Korea continues to launch missiles, develop its nuclear weapons program and engage in threatening rhetoric and behavior."
North Korea has hinted that it is ready to test an intercontinental ballistic missile capable of reaching the continental U.S., an early challenge for the Trump administration. "Any attack on the United States or our allies will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Mattis said.
It was, in effect, the first time the Trump administration had outlined its stance on North Korea's nuclear provocations.
"The message was targeted at both North Korea and hardliners in the U.S. who were increasingly calling for an attack on North Korea if Pyongyang were merely to test an intercontinental ballistic missile," said Tetsuo Kotani, chief researcher at the Japan Institute of International Affairs, a think tank. "Mattis made clear that the U.S. would only respond to North Korea if it were to attack the U.S., an ally or to use nuclear weapons," Kotani said.
In the meeting, the two sides agreed to deploy and operate the Terminal High Altitude Area Defense (THAAD) missile defense system in South Korea this year to counter the threat from the North, South Korea's Defense Ministry later said.
China has strongly opposed THAAD, seeing it as a threat to its own national security. Opposition parties in South Korea are also cool to deploying the system.
Mattis kicked off his trip to South Korea and Japan on Thursday, with a meeting with Seoul's acting president, Prime Minister Hwang Kyo-ahn.
Speaking about his first overseas visit -- to South Korea and Japan rather than the Middle East or Europe -- Mattis said the purpose was to "underscore America's priority commitment to our bilateral alliance" and make clear the administration's "full commitment" to defending South Korea's democracy.
Mattis promised a stronger alliance with South Korea, saying Washington would stand "shoulder to shoulder" with Seoul in response to the North Korean threat. Most of his meeting time was spent discussing Pyongyang, and he agreed on responding strongly to any new provocations by the North, such as test-launching an intercontinental ballistic missile. This came as a relief to a government in crisis since the impeachment of President Park Geun-hye.
As a candidate, U.S. President Donald Trump had suggested withdrawing American troops from South Korea if the country does not pay more to keep them here. He even hinted at possibly meeting with North Korean leader Kim Jong Un, fueling deep concerns in the South. Mattis did not touch on the cost issue Thursday and stressed the North's nuclear threat as a top security concern for the Trump administration.
Nicknamed "Mad Dog" Mattis, the former Marine has commanded troops in both Afghanistan and Iraq. He rose to the rank of general and headed the U.S. Central Command, directing forces in the Middle East, before retiring from the military.
With Secretary of State Rex Tillerson a former Exxon Mobil CEO lacking in foreign policy experience, Mattis is expected to take a leading role in Asian diplomacy.
Mattis meets with Japanese Prime Minister Shinzo Abe on Friday to discuss the security environment in East Asia. Tokyo is seeking reassurances that the disputed Senkaku Islands fall under Article 5 of the bilateral security treaty, under which the U.S. would help defend Japan in cases of military attack.
"We trust in U.S. involvement," Japanese Chief Cabinet Secretary Yoshihide Suga told reporters Thursday. "We want to be on the same page with the new Trump administration."
An American commitment to Article 5 will serve as a deterrent against China, which claims the Senkakus for itself as the Diaoyu. Beijing will likely exploit any cracks it sees in the Japan-U.S. alliance.
With Tillerson now confirmed as secretary of state, Japan is working to build a diplomatic and security pipeline to the new administration. The countries' defense chiefs will meet Saturday. Japanese Foreign Minister Fumio Kishida will accompany Abe next week to the U.S., where he could hold his first meeting with Tillerson.