"This is a very serious crime," Defense Minister Satoshi Morimoto told reporters. Morimoto said the case follows another sexual assault in August, and he indicated he was considering discussing the matter with U.S. Defense Secretary Leon Panetta.
Okinawan police arrested the two U.S. sailors on Tuesday. They were identified as Seaman Christopher Browning and Petty Officer 3rd Class Skyler Dozierwalker of the Fort Worth Naval Air Base in Texas. Both are 23. According to Japanese media reports, they had been drinking before they attacked the woman, in her 20s, who was on her way home before dawn Tuesday.
The two were in Japanese police custody, according to Okinawa Prefectural Police spokesman Takashi Shirado. Later Tuesday, police handed over investigation to prosecutors to decide whether to press formal charges.
The arrests sparked immediate anger on Okinawa, which hosts more than half of the 47,000 U.S. troops in Japan and has recently seen massive protests against plans to deploy the Marine Corps' MV-22 Osprey to a base there because of safety concerns.
Okinawa Gov. Kazuhiro Nakaima, who has been in Tokyo since earlier this week, said the United States should worry about the consequences of repeated crime by U.S. servicemen on the island, where people already resent their presence.
"It could damage Japan-U.S. security alliance unless there is dramatic improvement," Nakaima said. "We want the U.S. government and its military to take measures that are far severer than a disciplinary measure or something lenient like that."
Tensions between the U.S. military and their Okinawans hosts are endemic, and base-related crimes are a particularly sensitive issue.
Local opposition to the U.S. bases over noise, safety concerns and crime flared into mass protests after the 1995 rape of a schoolgirl by three American servicemen. That outcry eventually led to an agreement to close a major Marine airfield, but that plan has stalled for more than a decade over where a replacement facility should be located.
Concerned that anti-base sentiment on Okinawa could swell, Vice Foreign Minister Shuji Kira lodged a protest with U.S. Ambassador John Roos, who promised full cooperation with the investigation.
"The United States government is extremely concerned by recent allegations of misconduct by two individual United States service members," Roos said in a statement. "We are committed to cooperating fully with the Japanese authorities in their investigation. ... These allegations, given their seriousness, will continue to command my full personal attention."
He said he understands the anger in Okinawa and the rest of Japan and promised to "work our hearts out to earn the trust of the Okinawan people and the people of Japan."
Okinawa prefectural spokesman Susumu Matayoshi said the suspected rape "shocked all Okinawans and is unforgivable."