US Assistant Secretary of State Kurt Campbell told an audience in Tokyo of Washington's sorrow, the latest move to soothe anger at a time of heightened anti-US feeling on the island.

"I need to begin, if I may, with a heartfelt apology and (the expression of) deep commitment to Japanese friends," Campbell, the US point man on East Asia, said in a symposium on the Japan-US alliance in Tokyo.

"These incidents are profoundly, deeply regrettable and the United States takes them very seriously," he said.

The United States put all 47,000 military personnel, both in Okinawa and elsewhere, under an indefinite nighttime curfew in response to the alleged rape, following the arrest of two servicemen last week.

"We continue to work closely with Japanese colleagues to do everything possible to make clear our commitment to... our hosts in Okinawa who support American military engagement in Japan and in the region," Campbell said.

The rape case came amid already high tensions in Okinawa, which has seen angry demonstrations against the US deployment to the island of the tilt-rotor Osprey aircraft, which local activists charge has a poor safety record.

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 fuelled resentment against both Tokyo and Washington in Okinawa, which was under US control from 1945 to 1972.