Japanese Prime Minister Fumio Kishida has said the country has reached “a basic agreement” to impose restrictions on the US military amid growing concerns about a sharp rise in coronavirus cases.
Mr Kishida said American soldiers will stay on base “except when absolutely necessary”, which presumably means for emergencies or other security reasons.
Details of the deal are still being worked out, he said on Fuji TV, but the overall US-Japan security alliance remains unchanged.
Daily Covid-19 cases have surged recently in what medical experts call “the sixth wave”.
New infections jumped above 8,000 on Saturday, a four-month record.
A spokesman for US forces in Japan was not immediately available for comment on Mr Kishida’s latest remarks.
But Major Thomas R Barger has said Covid trends are being closely monitored among the ranks for “health protection and operational readiness”.
Okinawa, a south-western group of islands that house most of the 55,000 US troops in Japan, is among the three prefectures where new restrictions to curb the spread of infections came into force on Sunday.
The measures, which last until the end of the month, force restaurants to close early, at 8pm or 9pm, and some must stop serving alcohol.
Government-backed restrictions also took effect in Yamaguchi Prefecture, where Iwakuni base is located, and nearby Hiroshima.
The Hiroshima Peace Memorial Museum, which documents the US atomic bombing of Japan at the end of the Second World War, and Hiroshima Castle are both closed to visitors.
Other regions may order similar regulations if cases keep rising.
People have been warned to stay at home and avoid travel. Until recently, bars, shrines and shopping districts have been jam-packed with end-of-year shoppers and New Year travellers.
About 80% of the Japanese population have received two doses of a vaccine.
Boosters have barely started, with fewer than 1% receiving them, despite repeated promises by the government to speed up their rollout.
Japan has set up stringent border controls, barring most incoming travel except for returning residents and citizens.
The country has reported about 18,300 Covid-related deaths so far. In recent days, there have been only one or two deaths, and on some days zero.