MANILA (Reuters) - Philippine prosecutors issued subpoenas on Tuesday to question six U.S. soldiers accused of gang-raping a Filipino woman in a van after military exercises earlier this month.

Jovencito Zuno, the chief state prosecutor, said the suspects had 10 days to respond with counter affidavits and that he hoped an initial investigation would be completed in 60 days.

<##IMAGE##>The six soldiers, who have not been charged with any offence, remain in U.S. custody in the Philippines after their colleagues aboard the USS Essex left when the military exercises ended.

Manila is a major ally to Washington in the region and both sides say the alleged attack on a 22-year-old woman in the Subic Bay area on November 1 will do nothing to hurt relations.

Still, leftist and women"s groups have called for a visiting forces agreement between the two countries to be scrapped and the six U.S. soldiers to be brought under Philippine jurisdiction.

"We have already presented formally the complaint," Zuno said. "When we find sufficient evidence against the respondents and the case is filed in court and the court issues warrants of arrest that will be the time that we can demand custody."

Officials at the U.S. embassy in Manila pledged cooperation in the investigation and said the suspects would be made available for questioning but have given no indication whether the soldiers could be turned over to Philippine authorities.

"The visiting forces agreement was designed between our two nations exactly to deal with situations like this where there have been allegations of criminal misdeeds," Scott Bellard, the U.S. embassy"s acting chief of mission, told reporters.

The United States, a colonial ruler of the Philippines for nearly half a century, was ordered to close its last military base in the country in the early 1990s but continues to train and advise local troops fighting Muslim and communist insurgencies.