Asian stocks slipped on Wednesday but losses were capped by caution ahead of a U.S. Federal Reserve policy decision later in the day. The Fed is expected to keep interest rates unchanged this week and may struggle to convince skeptical investors it can tighten monetary policy before the end of the year in the face of U.S. and global economic headwinds. China stocks, however, for now shrugged off U.S.-China tensions over naval patrols in the South China Sea.