TOKYO, Jan 6 (Reuters) - U.S. Forces in Japan (USFJ) on Thursday said they have introduced stricter measures to tackle an increase in COVID-19 cases that have raised concerns that cases may spread to communities around military facilities in the country. The USFJ is "establishing more stringent mitigation measures in a further effort to prevent virus transmission," it said in a press release. The measures include requiring U.S. military personnel to wear masks off base and for stricter testing mandates.