WASHINGTON, Aug 18 (Reuters) - The U.S. Transportation Department on Wednesday said it will limit some Chinese air carrier flights to 40% passenger capacity for four weeks after the Chinese government imposed similar limits on four United Airlines flights . China told United on Aug. 6 that it was imposing sanctions after it alleged five passengers who traveled from San Francisco to Shanghai tested positive for COVID-19 on July 21. The U.S. order said the department will limit over a four-week period each of four Chinese carriers providing scheduled China-U.S. passenger service to a 40% passenger load factor on a single China-U.S. flight. Reporting by David Shepardson; editing by Diane Craft Our Standards: The Thomson Reuters Trust Principles.