The alleged rape of a young girl has triggered protests in Japan

Under the curfew, which began early on Wednesday, troops' movements will be restricted to allow them a "period of reflection", a military statement said.

Thousands of US troops are based in Okinawa, Japan's southern-most island.

Many Okinawans dislike the US presence. The recent alleged rape of a teenager by a US marine has caused outrage.

In 1995 there were mass protests in Okinawa after US troops gang-raped a 12-year-old girl - an incident which raised concerns for the future of the US-Japan security alliance.

Both US and Japanese officials are working to prevent a similar outpouring of anger, analysts say.

'Reflection'

The order for the curfew - which is indefinite and also applies to troops' relatives - came from the top US commander in Okinawa, Lt-Gen Richard Zilmer.

"Active duty service members on Okinawa will be limited to their place of duty or employment, worship, education, or medical or dental treatment" as they enter a "period of reflection", a military statement said.

The restrictions would give troops "an opportunity to further review procedures and orders that govern the discipline and conduct of all US service members serving in Okinawa", it said.

In a separate announcement, the US military said that troops across Japan would hold a "day of reflection" on Friday over the recent incidents.

Last week, US Marine Staff Sgt Tyrone Hadnott was arrested for allegedly raping a 14-year-old girl in his car. He admits forcibly kissing her but denies rape.

Over the weekend, two more US troops were arrested for drink-driving and trespassing - causing Japan's top government spokesman to accuse the US military of a lack of discipline.

"I only have one thing to express and that is our true anger," Nobutaka Machimura said. "We demand serious self-reflection."

Japanese leaders would raise the issue with US Secretary of State Condoleezza Rice when she visited the country next week, he said.

US Ambassador Thomas Schieffer flew to Okinawa last week to express his concern over the alleged rape.

About 20,000 US troops - half of the US presence in Japan - are stationed in Okinawa as part of the decades old US-Japan security alliance.

The bases are strategically very significant because of their proximity to the Taiwan Strait.

Local residents complain of crime linked to the military presence, as well as noise and disruption caused by the bases - but local leaders say the bases are vital to the island's economy.