Image caption There have been many protests over the US military footprint, like this one over the Osprey deployment

Two US troops have been arrested over the alleged rape of a Japanese woman on the island of Okinawa.

The two men, identified as 23-year-old sailors, were detained by police on the southern island on Tuesday.

Japan's top spokesman called the incident "completely unacceptable" and the US ambassador said it was being taken very seriously.

The US bases some 26,000 troops on Okinawa, where there is opposition to the ongoing military presence.

The gang-rape in 1995 of a 12-year-old girl by three US service personnel sparked outrage on the island and led to moves to reduce the US military footprint there.

But the moves - which include closing a key air base and replacing it with a new base in the north of the island - are stalled amid entrenched opposition in Okinawa to the construction of the new base.

'Extremely concerned'

The two sailors are in the custody of Okinawan police after the alleged incident, which happened in the early hours of Tuesday.

In a statement, Ambassador John Roos said the US was "extremely concerned by recent allegations of misconduct" by two US service personnel.

"We are committed to cooperating fully with the Japanese authorities in their investigation."

Japan has lodged a protest with the US and demanded that stricter disciplinary measures be put in place.

Japan's Chief Cabinet Secretary Osamu Fujimura said that representatives from the defence ministry had told US officials that "the incident is completely unacceptable".

"They have strongly requested measures to increase discipline and prevent things like this from happening again," Mr Fujimura said.

For his part, Japanese Prime Minister Yoshihiko Noda told reporters: "This should never have been allowed to happen."

Okinawa Governor Hirokazu Nakaima has also met Japanese Defence Minister Satoshi Morimoto to protest against the incident, saying the alleged crime was "extremely heinous and despicable", Kyodo news agency reports.

Some observers have recently described Okinawa as a tinder box of resentment, says the BBC's Rupert Wingfield Hayes in Tokyo.

Okinawans are not only angry at the continued huge US military presence, but also at their perceived treatment as second-class citizens by their political masters in Tokyo, our correspondent adds.

About half of all US troops based in Japan are stationed in Okinawa, which lies south of Japan's main islands in the East China Sea.

Residents have long argued that the island hosts more than its fair share of US bases and complained of base-related crime as well as the amount of land allocated to the US military.

The US says it is planning to relocate several thousand marines off Okinawa, but has linked their departure to progress on the stalled base relocation plan.

The most recent protests in Okinawa came after the US deployed Osprey aircraft there. Residents say they have a poor safety record but the Japanese government says they are safe.