The US can't fully trust China until China acts by the rules, US Defense Secretary Patrick Shanahan said at Asia's premier defence forum Saturday morning in Singapore. He criticized China for its activities in the South China Sea.

The US accuses China on island-building, deploying advanced weapons systems to disputed areas, engaging in predatory economics and the alleged state-sponsored theft of military and civilian technology, CNN reports.

Read more: 'China will continue to grow and strengthen, countries have to accept' - Singapore PM

Beijing claims almost the entire 1.3 million square miles South China Sea as its sovereign territory as President Xi Jinping saying it will never give up "any inch of territory. US military officials, meanwhile, have vowed to continue enforcing a free and open Indo-Pacific.

"We can't ... continue to look the other way as countries use friendly rhetoric to distract from unfriendly acts," Patrick Shanahan said, adding that Washington had a "deeper and broader" network of allies and partners in the Indo-Pacific than China.

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"Behavior that erodes other nations' sovereignty and sows distrust of China's intentions must end," Shanahan said. While taking questions from the floor after his speech, he added: "We're not going to ignore Chinese behaviour. In the past, people have kind of tip-toed around that."