By David Alexander and Pete Sweeney WASHINGTON/SHANGHAI (Reuters) - A U.S. guided missile cruiser operating in international waters in the South China Sea was forced to take evasive action last week to avoid a collision with a Chinese warship maneuvering nearby, the U.S. Pacific Fleet said in a statement on Friday. The incident came as the USS Cowpens was operating near China's only aircraft carrier, the Liaoning, and at a time of heightened tensions in the region following Beijing's declaration of an Air Defense Identification Zone farther north in the East China Sea, a U.S. defense official said. The near miss was the most significant U.S.-China maritime incident in the South China Sea since 2009, said security expert Carl Thayer at the Australian Defense Force Academy. Heightened tensions over China's military assertiveness have raised concerns that an minor incident in disputed maritime waters, the South China Sea and East China Sea, could quickly escalate. Both Japan and China lay claim to islands in the East China Sea and have scrambled aircraft in recent months over the disputed seas and conducted naval patrols. China and several ASEAN nations have competing territorial claims in the South China Sea. The U.S. has raised the latest incident at a "high level" with the Chinese government, according to a State Department official quoted by the U.S. military's Stars and Stripes newspaper. In Beijing, the Chinese foreign and defense ministries have yet to respond to questions about the incident, while China's often-nationalistic on-line platforms were filling with debate about the near-miss. One poster demanded the Chinese navy follow-up by blazing an "independent sea lane" to Hawaii. Beijing routinely objects to U.S. military surveillance operations within its exclusive economic zone, while Washington insists the United States and other countries have the right to conduct routine operations in what it says are international waters. China deployed the Liaoning to the South China Sea just days after announcing a new air defense zone which covers air space around a group of tiny islands in the East China Sea that are administered by Japan but claimed by Beijing as well. The United States and its allies rejected the demand and have flown military aircraft into the zone. The Chinese carrier, which has yet to be fully armed and is still being used as a training platform, was flanked by escort ships including two destroyers and two frigates. Security expert Thayer said the incident was the most significant since five Chinese ships harassed a U.S. oceanographic research vessel, the USS Impeccable, in 2009, also in the South China Sea. "There have been hints of other incidents that both sides have apparently kept quiet but not this time," he said. The U.S. is determined to stand by its rights in international waters and is clearly expecting China to act accordingly. Additional reporting by Manuel Mogato in MANILA and Greg Torode in HONG KONG; Editing by Jim Loney, David Brunnstrom and Michael Perry.