TOKYO - A U.S. court martial sentenced a Marine to four years in prison on Friday for sexual abuse of a 14-year-old girl on the southern island of Okinawa, in a case that has sparked widespread public anger.

Many on the island, home to about half the nearly 50,000 U.S. military personnel in Japan, are calling for troop numbers to be cut back, citing concerns over crime, noise and pollution.

A string of incidents this year, including the killing of a taxi driver, have forced the United States, pacifist Japan's most important security ally since World War Two, to apologize and vow to raise standards of behavior.

Tyrone Hadnott, 38, was arrested by Japanese police on suspicion of rape following the incident in February, but released after the girl withdrew her complaint, a move some in Okinawa said was to escape public attention.

Hadnott was also given a dishonorable discharge at the half-day trial, a spokesman for Okinawa Marines said.

The case was a painful reminder for many of the gang rape of a 12-year-old girl by U.S. Marines in Okinawa in 1995, which provoked a wave of opposition to their presence.

Lt. Gen. Edward Rice, the commander of U.S. forces in Japan, reiterated on Friday that such crimes by servicemen should not be allowed to affect the security relationship between the two countries.

"I think this is an example of how we in the United States military will continue to hold our members accountable for their actions," he told a news conference in Tokyo.

"Even though he was not prosecuted under the Japanese judicial system, we found that there was enough evidence to prosecute him under the U.S. system," he added.

Rice said a survey last month of training programs aimed at preventing sexual assaults found that they were being carried out in line with U.S. government guidelines, adding that efforts would continue to improve awareness.

U.S. Forces agreed on Thursday to inform Japan promptly of the details of deserters, after a U.S. sailor who had gone missing was arrested for the murder of a Japanese taxi driver last month.

Rice also called on the Japanese to remember the strategic importance of U.S. bases on their soil.

"When you look at the larger issues at stake and the larger strategic landscape, it becomes very clear that the positive influence of these forces is something that is of great benefit to Japan and the other countries in the region," he said.

Eight thousand Marines are set to be moved from Okinawa to Guam under a realignment aimed at reducing the burden on the Japanese island. But local disagreements remain over the details of the plan, which also involves shifting the Futenma Marine base to a less populated part of the island.

Rice said he was optimistic that the plan would go ahead as scheduled by 2014.