U.S. Defense Secretary Jim Mattis said Saturday the U.S. cannot afford to ignore destabilizing moves by Iran, but has no plans to respond by increasing American military forces in the Middle East.

Mattis spoke at a joint news conference in Tokyo with his Japanese counterpart, Tomomi Inada. Mattis held a series of high-level meetings in Seoul and Tokyo this week on his first overseas trip as Pentagon chief. He was returning to Washington Saturday.

Asked about U.S. concern about China's militarization of artificial islands in the South China Sea, Mattis was critical of China's moves but said U.S. military action was not appropriate.

In an opening statement at his news conference, Mattis also explicitly stated that the Trump administration will stick to the previous U.S. stance that the U.S.-Japan security treaty applies to defending Japan's continued administration of the Senkaku islands in the East China Sea, which are contested by China.

In response, China's Foreign Ministry reasserted its claim of sovereignty over the tiny, uninhabited islands and called on the U.S. to cease "making wrong remarks" over the issue.

Saturday's news conference was Mattis' first since taking office Jan. 20. He is the first career military officer to serve as defense secretary since George C. Marshall, a former Army chief of staff, in 1950-51 during the Korean War. Mattis served 41 years in the Marine Corps, including a stint on Okinawa.

Mattis' task in South Korea and Japan was to assure each government that the Trump administration will stick by its treaty obligations. President Donald Trump had said during the campaign that the alliances were a bad deal for America because the allies do too little for U.S. security.

Mattis addressed this head-on.

"Japan has made noteworthy contributions to regional security and to the alliance, and the United States deeply appreciates Japan's contributions," he said. "But make no mistake: In my meeting with Japanese leaders, both our nations recognize that we must not be found complacent in the face of the challenges we face."

The U.S. has 28,500 troops in South Korea and about 50,000 in Japan.

On the Iran issue, Trump administration officials have said they were actively considering a "range of options," including economic measures and increased support for Iran's regional adversaries.

On Friday, the administration ordered sanctions against more than two dozen people and companies from the Persian Gulf to China in retaliation for Iran's recent ballistic missile test. Those targeted by the Treasury Department include Iranian, Lebanese, Emirati and Chinese individuals and firms involved in procuring ballistic missile technology for Iran.

The Trump White House has left unsaid whether military action would be part of intensifying pressure on Iran.

Iran "is the single biggest state sponsor of terrorism in the Middle East," Mattis said Saturday. "I think it is wise to make certain that Iran recognizes that what it is doing is getting the attention of a lot of people."

He added: "It does no good to ignore it. It does no good to dismiss it. At the same time, I don't see any need to increase the number of forces we have in the Middle East at this time."

During his tenure as commander of U.S. Central Command, which ended in 2013, Mattis was known to have pushed for more aggressive actions against Iran than the Obama White House would tolerate. He has called Iran the biggest threat in the Middle East.

One of the few specifics Mattis offered at his Senate confirmation hearing on combatting the Islamic State group in Syria was that he believed the campaign to take Raqqa should be accelerated. Since then he has said little on the subject.

The Obama administration had set a limit of about 500 US troops in Syria, a policy derived from then-President Barack Obama's resistance to getting drawn into another war in the Middle East.

U.S. special operations troops have been working in small groups with local Syrian opposition fighters, both Kurdish and Arab, as part of a strategy to gradually retake Raqqa, the Islamic State group's declared capital. The U.S. also has been conducting airstrikes.