Defense Secretary Jim Mattis (left) speaks with South Korean Defense Minister Han Min-koo before their meeting in Seoul on Friday. (Kim Min-Hee/AFP/Getty Images)

Defense Secretary Jim Mattis pledged Friday that the United States would live up to its security commitments, signaling a strong partnership with Asian allies while vowing a forceful response against North Korean aggression.

Mattis met with South Korean and Japanese leaders during his first overseas trip since becoming President Trump’s defense secretary last month.

Speaking in Seoul alongside South Korean Defense Minister Han Min-Koo, Mattis promised that the U.S. policy of defending allies was “ironclad.”

“The United States stands by its commitments, and we stand with our allies, the South Korean people,” he said.

In Tokyo later in the day, Mattis likewise sought to assuage concerns about the direction of Trump’s foreign policy, which has already broken with previous U.S. practice in important ways and rattled longtime allies.

During the presidential campaign, Trump called on Japan and South Korea to pay more for a U.S. military presence that has been a fundamental aspect of U.S. regional strategy for decades. There are almost 30,000 U.S. troops stationed in South Korea and about 50,000 in Japan.

Mattis, a retired Marine general whose previous assignments include working closely with Middle Eastern allies and overseeing coalition operations in Afghanistan, has departed from Trump’s apparent skepticism of traditional military alliances, giving a strong defense in his Senate confirmation hearing of the NATO organization and other partnerships.

Speaking before talks with Japanese Prime Minister Shinzo Abe, Mattis said: “I want there to be no misunderstanding during the transition in Washington. We stand firmly, 100 percent shoulder-to-shoulder with you and the Japanese people.”

Mattis cited a mutual defense pact with Japan, which requires the United States to respond in the event that Japan is attacked.

That clause, Mattis said, “is as real today as it was a year ago, five years ago, and as it will be in a year, 10 years from now.”

Mattis’s emphasis on the dangers emanating from Pyongyang appears to signal a re-examination of the U.S. approach to North Korea, as leader Kim Jong Un continues his program of missile and nuclear tests and threatens to debut an intercontinental ballistic missile capable of delivering a nuclear weapon to the United States.

Mattis sent a message to North Korea, which he blamed for “threatening rhetoric and behavior.”

“Any attack on the United States, or its allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming,” he said in Seoul.

The White House is reviewing policy on North Korea and a range of other foreign policy issues, officials said.

Two weeks after taking office, the Trump administration is already showing signs of a tougher stance on an array of foreign policy issues, including Iran’s use of ballistic missiles and its disagreement with Mexico over trade and border security.

Officials said the Trump administration is committed to moving ahead with installing the Terminal High-Altitude Area Defense system (THAAD), which is designed to protect against North Korean ballistic missiles, in South Korea.

South Korea’s current government has supported hosting the defensive measure, which is strongly opposed by China. But elections are expected in coming months, and it’s unclear whether the country’s next government will accept the controversial system.