The United States and China must cooperate on formulating a response to North Korea after the sinking of a South Korean warship blamed on Pyongyang, US Secretary of State Hillary Clinton said Monday. "North Korea is also a matter of urgent concern. Today we face another serious challenge provoked by the sinking of the South Korean ship," Clinton said at the start of two days of high-level talks with Chinese officials. "So we must work together to address this challenge and advance our shared objective of peace and stability on the Korean peninsula," she said. "We ask North Korea to stop its provocative behaviour, halt its policy of threats and belligerence towards its neighbours and take irreversible steps to fulfill its denuclearisation commitments and comply with international law."