Okinawa police arrested an apparently drunk US Marine on Sunday on suspicion of trespass amid growing anger on the southern Japanese island over military crimes and demands for stricter regulations for American troops.
The incident, the second after the US military had stepped up disciplinary steps last month, immediately triggered harsh reaction from the Okinawa government.
Police said 1st Lt Tomas Chanquet of the marine air station Futenma allegedly sneaked into a room through an unlocked door and slept until spotted by a resident who called police.
Sunday's arrest was especially inflammatory on Okinawa, where the emperor Akihito was visiting to attend a fisheries event.
"I'm too shocked to say anything. It's utterly ridiculous and extremely regrettable," Okinawa governor Hirokazu Nakaima told reporters. "I must lodge a strong protest to both the Japanese and US governments. They must do something more significant."
Prime minister Yoshihiko Noda is expected to raise the issue during talks Tuesday with President Barack Obama on the sidelines of the summit of south-east Asian countries in Phnom Penh, Cambodia, Kyodo News agency reported.
An alleged rape by two navy sailors last month enraged Okinawans and reignited deep-rooted anti-base sentiment on the island, home to more than half the 52,000 US troops in Japan.
The case led to a curfew on all troops in Japan, but two weeks later an American airman allegedly assaulted a teenager. Sunday's incident also raises questions over the effectiveness of the curfew and other disciplinary steps.
Japan has lodged a formal protest with the US embassy and military over the incident Sunday and demanded that they make sure the curfew is enforced.
Chanquet was apparently drunk when he entered the apartment, Okinawa police official Masahiko Gishi said. Police are investigating if he broke the curfew and was drinking off-base prior to the alleged trespassing.
On Friday, Okinawa's state assembly adopted a resolution protesting the two earlier cases, demanding tougher regulations and stepped-up efforts to reduce the number of troops and bases on the island. The resolution also called for a review of legal procedures for military suspects and efforts to streamline the US troop presence.
Okinawans have staged massive protests against the deployment in October of Osprey military aircraft despite opposition over safety concerns following two crashes elsewhere.
Local opposition to the US bases over noise, safety and crime flared into mass protests after the 1995 rape of a schoolgirl by three American servicemen. The outcry eventually led to an agreement to close the Futenma airfield, but the plan has stalled for more than a decade over where a replacement facility should be located.
