Washington wants to work with Beijing on denuclearizing North Korea, but cannot accept China's actions in the South China Sea, U.S. Defense Secretary James Mattis said on Saturday in a speech that reflected the complex relationship between the world's two largest economies.

Speaking at the Shangri-La Dialogue in Singapore, Mattis said President Donald Trump was encouraged by China's "renewed commitment to work with the international community" on ending the ballistic missile and nuclear weapons programs that North Korea shows no sign of abandoning.

Following the rogue nation's latest ballistic missile test last month, the U.N Security Council expanded sanctions against North Korean leader Kim Jong-un's regime on Friday while Trump's administration blacklisted several companies and people on Thursday for supporting Pyongyang's arms development.

Washington has urged Beijing to use its decades-old relationship with Pyongyang to apply more pressure on Kim – trade with China is a key source of revenue for North Korea's economy – amid a failure of international sanctions to halt Kim's missile tests.

Still, Trump's administration isn't holding back on criticizing the mainland.

"We cannot accept Chinese actions that impinge on the interests of the international community," Mathis said on Saturday, referring to the South China Sea conflict.