Despite having secured export licenses, some Chinese rare earth producers have stopped shipping these materials to the United States over fears of repercussions from China's authorities, sources familiar with the development told Reuters. The renewed standoff in rare earths comes weeks before Chinese President Xi Jinping is expected to visit Washington for a summit with U.S. President Donald Trump at the end of this month. A few Chinese suppliers have been refusing to export rare earths to the U.S. since the beginning of August, according to Reuters'…