BEIJING, May 15 (Reuters) - The death toll from China's massive earthquake could reach more than 50,000, the official Xinhua news agency reported on Thursday, quoting rescue headquarters.

Already some 20,000 are confirmed dead as a result of Monday's 7.9 magnitude quake, but tens of thousands more remain buried in rubble in areas of the worst-hit province of Sichuan that rescuers are only just beginning to reach. (Reporting by Guo Shipeng; Writing by Lindsay Beck)