WASHINGTON — The leader of Japan's southern island chain of Okinawa on Tuesday welcomed the "quick" US response to an alleged rape by servicemen of a local woman, saying Washington was taking the case seriously.

Governor Hirokazu Nakaima was visiting Washington for a previously planned symposium on Okinawa's heavy US military presence a week after two 23-year-old sailors were arrested for allegedly assaulting a woman on a street.

Nakaima, who had earlier called the purported crime "insane" and vowed to tell the United States that Okinawans were "fed up," praised the response he heard during his meetings in Washington.

"My impression is that they were very quick in their response," Nakaima told reporters.

"It's not just the response, but also, they're taking it seriously. That was apparent. The way they responded makes very clear they're taking it seriously," he said, while adding that he was awaiting further details on the case.

The United States put all 47,000 members of the US armed forces in Japan -- both in Okinawa and elsewhere -- under an indefinite nighttime curfew in response to the case.

The US ambassador to Japan, John Roos, pledged full cooperation with Japanese authorities and said that he shared the anger over the incident.

The alleged rape came amid already high tensions in Okinawa, which recently saw demonstrations against the US deployment to the island of the Osprey aircraft, which local activists charge has a poor safety record.

The protests against the Osprey have been among the largest in Okinawa since 1995, when tens of thousands took to the streets urging a smaller US footprint after three soldiers were arrested for the gang-rape of a 12-year-old girl.

Okinawa is home to around half of the US forces stationed in Japan under a treaty signed after World War II, when Tokyo was stripped of the right to wage war.

The heavy troop presence has long fueled resentment against both Tokyo and Washington in Okinawa, a former kingdom that was under US control from 1945 to 1972.

The 1995 gang-rape led the US military to ramp up cultural sensitivity training for troops in Okinawa. But the two troops arrested last week, Christopher Browning and Skyler Dozier Walker, were based in Texas and allegedly only on a brief mission to Okinawa.