BEIJING - U.S. Secretary of State Hillary Rodham Clinton urged China on Monday to work with the United States to co-ordinate a response to the sinking of a South Korean warship blamed on North Korea.

Opening two days of high-level U.S.-China talks in Beijing, Clinton said North Korea must be held to account for the incident, which international investigators have determined was caused by a torpedo fired from a North Korean submarine. China is North Korea's main ally and has thus far remained neutral on the investigation.

In Seoul, South Korean President Lee Myung-bak said his country will take the case to the U.N. Security Council on Monday. U.S. officials say they have more work to do to convince China that North Korea was responsible for the ship sinking.

Clinton called the situation with North Korea a "matter of urgent concern."

"Today, we face another serious challenge provoked by the sinking of the South Korean ship. So we must work together ... to address this challenge and advance our shared objectives 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 toward its neighbours and take irreversible steps to fulfil its denuclearization commitments and comply with international law," Clinton said.

She also called on China to continue work with the United States and other members of the U.N. Security Council to draft new sanctions on Iran to press it to come clean on its nuclear program. She said "the burden is on Iran" to prove its nuclear program is peaceful and avert fresh penalties.

Clinton was joined onstage at China's Great Hall of the People by Treasury Secretary Timothy Geithner and Chinese Vice Premier Wang Qishan and State Councilor Dai Bingguo, all of whom spoke. Neither Wang nor Dai specifically mentioned North Korea or Iran, but Dai made clear in that China would not support any attempt to provoke conflict.

"No attempt to stir up confrontation and stage war, be it a hot war, a cold war or even a warm war, will be popular in today's world," he said. "Nor will such an attempt lead to anywhere."