TOKYO, Jan 6 (Reuters) - U.S. military bases in Japan introduced stricter measures on Thursday to tackle an increase in COVID-19 cases as the government expressed grave concern over the outbreak. U.S. Forces Japan said it is "establishing more stringent mitigation measures in a further effort to prevent virus transmission." The measures include requiring U.S. military personnel to wear masks off base and for stricter testing mandates, it said in a press release. Earlier, Japanese Foreign Minister Yoshimasa Hayashi requested U.S. Secretary of State Antony Blinken that U.S. service members be restricted from leaving base, Chief Cabinet Secretary Hirokazu Matsuno told reporters. Matsuno said the Japanese government was "gravely concerned" about COVID infections at U.S. bases. The southern prefecture of Okinawa, host to 70% of U.S. military facilities in Japan, leads the country in new infections and requested quasi-emergency measures from the central government on Thursday. Japan halted entry of almost all foreign travellers in late November after Omicron was named a variant of concern by the World Health Organisation. But the U.S. military has been able to move staff in and out of the country under a separate testing and quarantine regime, and concerns about escape of the virus into the general public have rankled residents near bases. New COVID-19 cases more than doubled in Okinawa on Wednesday over the previous day in a surge that Governor Denny Tamaki has said is the result of Omicron. A U.S. Marine Corps station in the prefecture of Yamaguchi, in western Japan, said it discovered 115 new cases on Wednesday, adding to 182 the previous day.