Japan has lodged a protest at the United States embassy in Tokyo over at least two alleged sexual assault cases involving American service members in the southern Japanese island of Okinawa that were only recently made public.

Japan’s Vice Foreign Minister Masataka Okano met the US ambassador to Japan, Rahm Emanuel, on Friday, requesting disciplinary and preventive measures over the two attacks, which occurred within months of each other in December and May.

“Criminal cases and accidents by US military personnel cause great anxiety to local residents, and they should never occur in the first place,” Chief Cabinet Secretary Yoshimasa Hayashi told reporters.

Prosecutors in Naha, the capital of Okinawa, had earlier this month charged a 21-year-old US Marine Corps member with non-consensual sex and assault, allegedly committed in May, Hayashi said.

An Okinawa police spokesman said the woman had been “bitten in the mouth” and had taken two weeks to fully recover. Media reports said she had also been choked.

The case came to light just days after it emerged that a 25-year-old US airman in Okinawa had been charged in March with raping a teenage girl three months earlier.

Brigadier General Nicholas Evans, commander of the 18th Wing at Kadena Air Base in Okinawa, said on Thursday that he was “deeply concerned” about the “severity” of the allegations. “I regret the anxiety this has caused,” he said.

He promised the US military will fully cooperate with the investigation by the local authorities and the courts.

Stoking tensions

Both cases have caused outrage, stoking tensions among residents over US military bases in the region. Okinawa residents have long complained about accidents and crime related to the bases, expressing anger over the lack of disclosure.

The case involving the teenager is a reminder to many Okinawans of the rape in 1995 of a 12-year-old girl by three US service members, which prompted large protests against the heavy US troop presence on the island.

It led to a 1996 agreement between Japan and the US on the closure of a key US air station, though the plan has been delayed due to protests at the relocation site on another part of the island.

Some 50,000 US troops are deployed in Japan under a bilateral security pact, about half of them in Okinawa. The island’s strategic role is seen as increasingly important for the Japan-US military alliance in the face of growing tensions with China.