An arrest warrant was issued for a U.S. sailor being held by the American military in the killing of a Japanese woman, a media report said Saturday.

The Kyodo News agency reported that a court issued an arrest warrant for the sailor on robbery-murder charges.

The sailor, who was not identified, was being held at the U.S. Navy base in Yokosuka about 30 miles southwest of Tokyo pending an investigation into the killing, according to the U.S. Naval Forces Japan.

Police officials were not immediately available to confirm the report Saturday.

The suspect admitted killing Yoshie Sato, 56, during questioning Friday, according to Tsuneo Kosuge, a spokesman for Kanagawa Prefectural Police. Sato was found beaten and unconscious in Yokosuka on Tuesday, and later died of internal bleeding.

The Japanese government plans to ask the U.S. side to transfer custody of the suspect to police, Kyodo said. But Hiroyoshi Ichikawa, another Kanagawa police official, said he could not confirm the report.

If the U.S. agrees to the transfer, police are expected to arrest the serviceman as early as Saturday, it said. Under a U.S.-Japan agreement, the Navy would have to hand over the sailor if Japanese authorities requested it.

The Navy said it was cooperating with Japanese police, and had imposed a temporary curfew requiring Navy personnel to be back on base by midnight until Monday.

The case risks further inflaming local opposition to plans to build an American military airstrip in the southern island of Okinawa, and base a U.S. nuclear-powered warship at Yokosuka for the first time.

Reflecting the sensitivity of the case, the U.S. Embassy issued a statement Friday expressing regret for the crime.

"The U.S. military and the American people are deeply shocked and saddened by this event," U.S. Ambassador Thomas Schieffer said in a written statement.

In 1995, an uproar over the rape of a 12-year-old girl by three U.S. servicemen on Japan's southern island of Okinawa triggered massive protests and led to the relocation of an air base to a less densely populated part of the prefecture.

The rape case also resulted in an agreement with the U.S. military that it would hand over American suspects in serious crimes to Japanese authorities for pre-indictment investigation.

About 50,000 U.S. troops are stationed in Japan under a joint security pact, but Tokyo and Washington agreed in October to move 7,000 Marines from Okinawa to the U.S. territory of Guam, and shift within Japan some of the remaining troops.