ABOARD A MILITARY AIRCRAFT — Defense Secretary Jim Mattis said Tuesday that the U.S. would confront behavior out of step with international law, in response to a question about U.S. defense policy towards China’s increasing deployment of military weapons on territories in the South China Sea that are claimed by multiple nations.

“We are going out of our way to cooperate with Pacific nations. That’s the way we do business in the world, but we’re also going to confront what we believe is out of step with international law, out of step with international tribunals that have spoken on the issue,” he said.

Mattis made the remarks to a group of reporters traveling with him, including from Breitbart News, to Hawaii for a change of command ceremony at U.S. Pacific Command from outgoing commander Adm. Harry Harris to Adm. Philip Davidson on Wednesday.

He will then head to Singapore for the annual Shangri-La defense forum, where he will meet with a number of regional defense leaders and experts. A key topic expected to be discussed  will be how to confront China’s increasing militarization of the South China Sea.

The U.S. took the dramatic step last week of disinviting China from a major U.S.-led biennial naval exercise in the Pacific Ocean known as Rim of the Pacific (RIMPAC) this year, after China deployed anti-ship missiles, surface-to-air missile systems, and electronic jammers to the land features in the South China Sea. China also landed a bomber at Woody Island.

The Pentagon first invited China to attend in 2014, in a gesture of goodwill and improving U.S.-Sino military relations. The disinvite was described as an “initial response to China’s continued militarization of the South China Sea.”

“There had been a promise in 2015, by [Chinese President Xi Jinping], in the Rose Garden at a White House meeting, where they stated they would not be militarizing the Spratly Islands. We have seen in the last month they’ve done exactly that, moving weaponry in that was never there before,” Mattis said.

Over the weekend, the Pentagon also sailed two destroyers through the South China Sea as part of a Freedom of Navigation Operation (FONOP) aimed at keeping the waters open for international commerce. The FONOP angered China, which considers the body of water as part of its domain.

Mattis said the U.S. will continue those operations.

“You’ll notice that there’s only one country that seems to take active steps to rebuff them or state their resentment of them. But it’s international waters, and a lot of nations want to see freedom of navigation, so we’ll continue that,” he said.

He said U.S. diplomats are “robustly engaged” on the issue, and that there is growing international concern.

“Concerns have come to me not from American government circles but also from foreign navies that are also concerned, very concerned about this continuing militarization of features in the South China Sea,” he said.

He contrasted the U.S. Navy’s FONOPs with opaque Chinese behavior.

“There’s a very steady drumbeat of freedom of navigation operations. We’ve very open about them, and report them,” he said.