US President Barack Obama and Chinese Premier Li Keqiang agreed on Monday to step up cooperation in the United Nations Security Council and in law-enforcement channels after North Korea's fifth nuclear test, the White House said. UN diplomats say China and the United States have started discussions on a possible UN sanctions resolution in response to the test earlier this month, but Beijing has not said directly whether it will support tougher steps against North Korea. Obama met Li on the sidelines of the annual United Nations General Assembly session in New York. 'Both leaders condemned North Korea’s September 9 nuclear test and resolved to strengthen coordination in achieving the denuclearization of the Korean Peninsula, including by invigorating cooperation in the United Nations Security Council and in law enforcement channels on North Korea,' a White House statement said. China is isolated North Korea's most important diplomatic backer and its biggest trading partner. It has been angered by Pyongyang's repeated nuclear and missile tests and signed on to increasingly tough UN sanctions, but it has said it believes such steps are not the ultimate answer and called for a return to talks with North Korea. Chinese Foreign Minister Wang Yi told his Japanese counterpart last week China opposes 'unhelpful' unilateral sanctions on North Korea but will work within the United Nations to formulate a necessary response. Washington has pressed Beijing to do more to rein in North Korea. The United States has said it is willing to negotiate with the North if the country commits to get rid of its nuclear weapons, which Pyongyang has refused to do. The White House said Obama and Li also discussed the US-China economic relationship and its importance to the global economy. 'The President encouraged China to accelerate its continuing efforts to address industrial excess capacity, foster an environment conducive to innovation, and advance an orderly transition to a market-determined exchange rate,' the statement said. It said Obama also urged Beijing to establish a level playing field for all firms to compete fairly in China and that he and Li discussed the importance of achieving progress in negotiation of a US-China Bilateral Investment Treaty and of a World Trade Organization Environmental Goods Agreement. The statement added that the two leaders discussed climate issues and pledged to continue working toward bringing the Paris agreement into force as early as possible and to reach a market-based measure to reduce international aviation emissions.