US Defence Secretary James Mattis has warned that the US will not accept China's militarisation of man-made islands in the South China Sea. Speaking at a security conference in Singapore today, he said, such moves undermined regional stability. China's territorial claims in the resource-rich South China Sea are contested by several nations. <br/><br/> At the same time, Gen Mattis praised Beijing's efforts to restrain North Korea's missile and nuclear activity. His comments came shortly after the UN Security Council expanded targeted sanctions against North Korea in response to a series of missile tests conducted this year. The council voted unanimously to back the sanctions after weeks of negotiations between the US and China.