WASHINGTON (Reuters) - The U.S. military said on Thursday it would soon announce new measures to "ensure responsible behavior" by American troops on Japan's southern island of Okinawa, after the suspected rape of a Japanese woman threatened to strain ties between the allies.

The incident, which led to the arrests on Tuesday of two U.S. servicemen, has thrown new scrutiny on the behavior of the large contingent of American military troops on the island, located in the East China Sea close to mainland China.

Okinawa, occupied by the United States from 1945 to 1972, accounts for less than 1 percent of Japan's total land but hosts three-quarters of the U.S. military facilities in the country in terms of land area. It also plays host to more than half the 47,000 American troops stationed in Japan.

After the arrests, Japanese Defense Minister Satoshi Morimoto vowed to press the United States for measures for stricter discipline. Okinawa Governor Hirokazu Nakaima described the incident as "madness."

Pentagon spokesman George Little said the U.S. military was working closely with Japanese authorities investigating the incident and added in a statement, "We deeply regret any grief and trauma the victim may have endured."

"We are also examining and will soon announce a package of measures to ensure responsible behavior and to demonstrate our commitment to maintaining positive relationships with the local communities that host our forces," Little said.

The Pentagon declined to elaborate on the measures being developed.

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.

Outrage over the latest case of suspected rape comes 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 Japanese concerns about their safety.

(Reporting by Phil Stewart; Editing by Peter Cooney)