China’s placement of weapons systems on manmade islands in the South China Sea is designed to intimidate and coerce others in the region, the US defence secretary James Mattis has said, laying out a sharp criticism of Beijing at an international security forum.

Speaking at the Singapore summit, he warned that America’s recent move to disinvite China from a multinational naval exercise this summer was an “initial response” to the militarisation of the islands.

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And, in response to a question, he said that “much larger consequences” were possible in the future if China did not find a way to work more collaboratively with others in the region.

The US, he said, remained committed to ensuring free and open transit in the region.

“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, referring to the recent deployment of anti-ship missiles, surface-to-air missiles, electronic jammers and other equipment on the Spratly Islands, and the landing of bomber aircraft at Woody Island.

Mattis also struck at one of the key, longstanding disputes between the US and China, telling the conference that America will continue to provide defence equipment and services to Taiwan and oppose any effort to alter the status quo. China claims the self-governing island as its own territory to be brought under its control by force if necessary.

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But in a quick pivot, he said the US welcomed cooperation with China “wherever possible” and announced that he had accepted Beijing’s invitation to visit there soon. It remains to be seen if that invitation will stand after this conference.

The Pentagon leader’s comments at the annual Shangri-La Dialogue came in the wake of a tumultuous few weeks between the US and China.

Last week the US withdrew an invitation for Beijing to participate in an exercise known as Rim of the Pacific. China had participated in the exercise in 2014 and 2016.

The Pentagon said the decision to disinvite the Chinese navy was triggered by what it called strong evidence that China has deployed weapons systems on the islands, and called on China to remove them. China says it is within its rights to build up defences on islands in the South China Sea that it believes are its sovereign territory.

China’s activities, Mattis said in his speech Saturday, stand “in stark contrast to the openness of what our strategy promotes. It calls into question China’s broader goals.”

In recent years the US had sought to stabilise military relations with China, but the militarisation of the islands has been a persistent point of conflict. Many nations fear that Beijing will use the construction on the islands to extend its military reach and potentially try to restrict navigation in the South China Sea.

China has not sent high-level officials to the three-day meeting, in an apparent attempt to deflect attention from its campaign to expand its sovereignty across virtually the entire South China Sea.

Mattis made clear that the US does not expect nations to choose between the US and China, adding that Beijing should have a voice in shaping the region, while allies have a voice in shaping China’s role.

The US, he said, “will continue to pursue a constructive, results-oriented relationship with China, cooperation when possible, and competing vigorously where we must”.