TOKYO — Two U.S. servicemen were arrested on Japan's southern island of Okinawa on suspicion of raping a Japanese woman, police said on Wednesday, a case that could again strain Tokyo's ties with its closest ally, Washington.

The arrests come at a time when public opinion in Okinawa is at odds with Tokyo for allowing the U.S. deployment of Osprey hybrid aircraft on the island despite lingering concerns about their safety.

Tuesday's arrests also coincide with a sharp deterioration in Japan's relations with China over a disputed East China Sea island chain that makes it strategically important for Tokyo to reaffirm its alliance with the United States.

Okinawa is major centre for the U.S. military based in Japan.

Friction over U.S. bases on Okinawa intensified after the 1995 gang rape of a 12-year-old Japanese schoolgirl by three U.S. servicemen. The case sparked widespread protests by Okinawans, who had long resented the American presence due to crime, noise and deadly accidents.

"I feel strong anger and indignation," Japanese Defense Minister Satoshi Morimoto told Okinawa Gov. Hirokazu Nakaima, who described the incident as "madness".

"I will press the United States for measures to implement stricter discipline," Morimoto said.

U.S. ambassador to Japan John Roos said in a statement that his government was extremely concerned by the incident and was committed to cooperating fully with the Japanese authorities in their investigation.

"I am also in close contact with the commander, U.S. Forces Japan. These allegations, given their seriousness, will continue to command my full personal attention," Roos said.

The two U.S. servicemen are suspected of raping a woman early on Tuesday morning in central Okinawa, an Okinawa police spokesman said.

The case has been sent to Okinawa prosecutors, another police official said.