ABOARD A U.S. MILITARY AIRCRAFT (Reuters) - U.S. Defense Secretary Jim Mattis said on Tuesday that the United States would continue to confront what Washington sees as China’s militarization of islands in the South China Sea, despite drawing condemnation from Beijing for an operation in the region over the weekend.

U.S. Defense Secretary Jim Mattis testifies before a Senate Armed Services Committee hearing on the “Defense Department budget posture in review of the Defense Authorization Request for FY2019 and the Future Years Defense Program” on Capitol Hill in Washington, U.S., April 26, 2018. REUTERS/Aaron P. Bernstein

Reuters first reported that two U.S. Navy warships sailed near South China Sea islands claimed by China on Sunday, even as President Donald Trump seeks Chinese cooperation on North Korea.

The operation, known as “freedom of navigation,” was the latest attempt to counter what Washington sees as Beijing’s efforts to limit freedom of navigation in the strategic waters, where Chinese, Japanese and some Southeast Asian navies operate.

China expressed its anger, saying it had sent ships and aircraft to warn the U.S. warships to leave.

“You’ll notice there is only one country that seems to take active steps to rebuff them or state their resentment (to) them, but it’s international waters and a lot of nations want to see freedom of navigation,” Mattis told reporters while enroute to Hawaii, where he will oversee a change of command for U.S. Pacific Command.

While the Sunday operation had been planned months in advance, and similar operations have become routine, it comes at a particularly sensitive time and just days after the Pentagon disinvited China from a major U.S.-hosted naval drill.

Critics have said these operations have little impact on Chinese behavior and are largely symbolic.

Pentagon officials have long complained that China has not been candid enough about its rapid military build-up and using South China Sea islands to gather intelligence.

Recent satellite photographs showed China appeared to have deployed truck-mounted surface-to-air missiles or anti-ship cruise missiles at Woody Island.

Earlier this month, China’s air force landed bombers on disputed islands and reefs in the South China Sea as part of a training exercise in the region.[L3N1SR08Q]

“When they (Chinese) do things that are opaque to the rest of us, then we cannot cooperate in areas that we would otherwise cooperate in,” Mattis said.

Mattis said U.S. diplomats were engaged on the issue and he had heard concerns about Chinese actions not just from within the United States government, but also from regional allies.

He will have strong words for China when he travels to Singapore for the Shangri-la dialogue, a security forum, later this week.

China’s claims in the South China Sea, through which about $5 trillion in shipborne trade passes each year, are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.