SINGAPORE — U.S. Defense Secretary Jim Mattis on Saturday raised the prospect of additional American steps against China if its “militarization” of the South China Sea keeps apace. He said Beijing was intimidating and coercing others in the region by putting weapons systems on manmade islands.

President Donald Trump, backing the remarks by his Pentagon chief at an international security forum in Singapore, said on Twitter: “Very surprised that China would be doing this?”

Mattis said the Trump administration’s recent decision to disinvite China from a multinational naval exercise this summer was an “initial response” to Beijing’s island activity. Mattis called the U.S. action a “relatively small consequence. I believe there are much larger consequences in the future.”

China’s reliance on military muscle to achieve its goals “is not a way to make long-term collaboration the rule of the road in a region that’s important to China’s future,” Mattis said, when asked to elaborate.

“There are consequences that will continue to come home to roost, so to speak, with China, if they don’t find a way to work more collaboratively with all of the nations who have interests,” he said.

From left, U.S. Defense Secretary Jim Mattis, Japan's Defense Minister Itsunori Onodera and China's People's Liberation Army's Academy of Military Science Deputy President He Lei pose for photos at a ministerial roundtable on the sidelines of the 17th International Institute for Strategic Studies (IISS) Shangri-la Dialogue, an annual defense and security forum in Asia, in Singapore, Saturday, June 2, 2018, in Singapore. (Yong Teck Lim/AP)
From left, U.S. Defense Secretary Jim Mattis, Japan's Defense Minister Itsunori Onodera and China's People's Liberation Army's Academy of Military Science Deputy President He Lei pose for photos at a ministerial roundtable on the sidelines of the 17th International Institute for Strategic Studies (IISS) Shangri-la Dialogue, an annual defense and security forum in Asia, in Singapore, Saturday, June 2, 2018, in Singapore. (Yong Teck Lim/AP)

The bluster comes at a delicate time in U.S.-China relations, as the Trump administration is warily asking for China’s help with North Korea ahead of a June 12 summit, while also threatening a trade war. Trump renewed his threat to hike tariffs on Chinese goods, just days before his commerce secretary’s arrived in Beijing on Saturday for trade talks.

Mattis said there was little doubt about Beijing’s intentions.

“Despite China’s claims to the contrary, the placement of these weapons systems is tied directly to military use for the purposes of intimidation and coercion,” Mattis said at the annual Shangri-La Dialogue.

China recently has deployed anti-ship missiles, surface-to-air missiles, electronic jammers and other equipment on the Spratly Islands, and landed a bomber aircraft at Woody Island.