Cases have nosedived in recent months in Japan, which has seen a comparatively low 18,300 Covid deaths during the pandemic.

But in recent days, 215 people linked with the US base Camp Hansen in Japan's southern Okinawa region have tested positive, Foreign Minister Yoshimasa Hayashi said.

Hayashi said he had voiced "strong regret" to the commander of US Forces Japan, Lieutenant General Ricky Rupp, over anti-infection procedures for American military arrivals.

Japan's borders are closed to almost all foreign nationals. Incoming travellers must get tested before departure and on arrival, then isolate for two weeks at home or in hotels.

Hayashi said, however, the US military was testing soldiers only three to five days after they landed, with newly arrived troops allowed to move freely inside their base.

"These rules are not consistent with the Japanese rules," Hayashi told reporters in Tokyo, saying he had been instructed by Prime Minister Fumio Kishida to voice his concern to Rupp.

It was unclear how many in the Okinawa cluster have the fast-spreading Omicron virus variant, which accounted for nearly three-quarters of new US cases in the past week.

While the American military was tightening its rules, Hayashi said he had demanded strict enforcement.

Rupp told Hayashi he regarded the situation at Camp Hansen seriously and promised to increase testing of those entering Japan, among other measures, according to the foreign minister.

There are around 20,000 US Marines in Okinawa, along with thousands more troops from other American military services.

Last year, two US Marine bases in Okinawa -- including Camp Hansen -- were put into lockdown and restrictions imposed at other bases in the region, with local officials criticising the US military's containment efforts.