"President Barack Obama's push for China to release an imprisoned Nobel Peace Prize laureate and rising economic and trade friction could aggravate U.S. efforts to win crucial Chinese cooperation on global hot spots. Ever-delicate U.S.-China relations had seemed to be warming, with the countries agreeing recently to end an eight-month freeze on military exchanges. But Obama's praise Friday for Chinese dissident Liu Xiaobo's Nobel award will likely further rattle China at a time when the United States is stepping up pressure on Beijing over a currency policy Washington blames for job losses in the United States. This recent swing from calls for cooperation to criticism is typical of a complicated relationship that both countries call important for world stability. U.S. officials are trying, with varying success, to press China on economic and human rights matters without jeopardizing Chinese support on Iranian and North Korean nuclear standoffs, climate change and other difficult issues. The Obama administration says the relationship is mature enough to weather disagreements and to engage in blunt discussions. But Beijing, wary of appearing weak at a time of rising nationalism and deep social turmoil, often bristles at what it views as U.S. interference. In a statement released hours after Liu was awarded the Nobel, Obama praised the dissident as an ""eloquent and courageous"" supporter of human rights and democracy ""who has sacrificed his freedom for his beliefs."""