"It is therefore imperative that we do our part each of us to fulfill our obligations and work together to support our shared goal of denuclearisation on the Korean Peninsula," Mattis said in a policy speech at the Shangri-La defence summit in Singapore.
Pyongyang on Monday test-fired another rocket, the latest in a series of launches and atomic tests that have ratcheted up tensions over its quest to develop weapons capable of hitting the United States -- something President Donald Trump has said "won't happen".
"The regime's actions are manifestly illegal under international law," Mattis said.
"There is a strong international consensus that the current situation cannot continue. China's declared policy of a denuclearised Korean Peninsula is our policy as well, and also that of Japan and the Republic of Korea."
Trump -- who frequently denounced China on the campaign trail -- has turned to Beijing to help rein in North Korea's weapons programme.
Mattis's challenge on his visit is to reassure allies that America can apply pressure on China over its claims in the South China Sea, while at the same time convincing Beijing that controlling North Korea is in its own security interests.
"The Trump administration is encouraged by China's renewed commitment to work with the international community toward denuclearisation," Mattis said.
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