Mattis also struck at one of the key, longstanding disputes between the US and China, telling the conference that America will continue to provide defense 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.

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

As expected, the Pentagon chief gave only a brief mention of the ongoing negotiations for an historic summit later this month between President Donald Trump and North Korean leader Kim Jong-Un. Trump announced Friday that the nuclear-weapons summit he had canceled with North Korea's Kim Jong Un is back on. The summit is expected to be here in Singapore.

Mattis said the Pentagon will "hold the line" and support the diplomatic effort to secure the "complete, verifiable, and irreversible denuclearization of the Korean Peninsula."

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 the exercise known as Rim of the Pacific. China had participated in the exercise known as Rim of the Pacific 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 defenses 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."

Then on Sunday, two US warships sailed close to the

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Paracel Islands, which lie north of the Spratlys, the latest freedom of navigation operation designed to challenge Beijing's claims. China protested the maneuver.

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's delegation at the conference is being led by Lt-Gen He Lei, vice-president of the People's Liberation Army's Academy of Military Science. 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".