Prosecutors in a rape trial involving four U.S. Marines accused the U.S. Embassy on Wednesday of delaying the proceedings by refusing to allow American Navy investigators to testify.

The judge in the case ordered the embassy to explain the situation after a U.S. Navy investigator who testified for the prosecution Monday was barred from appearing again Wednesday.

The U.S. government said Thursday diplomatic immunity prevented American Navy investigators from testifying. "There are issues of diplomatic immunity ... concerning the appearance and testimony of diplomatic personnel in court," the U.S. Embassy said in a statement.

CountryWatch: Philippines

The statement said the U.S. government has worked with Philippine authorities under the Visiting Forces Agreement throughout the investigation and will continue to do so until the end of proceedings.

Prosecutors allege Lance Cpl. Smith raped a 22-year-old Filipino woman on Nov. 1, while the others — Lance Cpl. Keith Silkwood, Lance Cpl. Dominic Duplantis and Staff Sgt. Chad Carpentier — cheered him on.

The rape charges are punishable by up to 40 years in prison. The Marines have refused to answer the charges, and the judge entered a plea of innocent for them.

U.S. Navy Criminal Investigation Service agent Ronald Veltz testified Monday that the driver of a van in which the alleged rape occurred told him the servicemen were indeed with the woman that night.

Veltz had been scheduled for cross-examination Wednesday, and a second Navy investigator was expected to testify.

But prosecutor Emilie Delos Santos said the U.S. Embassy told the Philippine Foreign Affairs Department that the investigators had diplomatic immunity and were not authorized by Washington to testify in the case.

"They are delaying the proceedings in this case," Delos Santos told the court.

The session adjourned after Judge Benjamin Pozon ordered the U.S. Embassy to explain why it was invoking diplomatic immunity for Navy investigators.

"This constitutes an affront to the powers of this court and the sovereignty of this country," Evalyn Ursua, a lawyer for the woman, told the court.

"The U.S. government is in violation of its commitment under the Visiting Forces Agreement. This is an attempt to obstruct justice," Ursua said.

The 1999 agreement has a provision that lets U.S. authorities hold servicemen facing a criminal case, and the U.S. Embassy has refused to turn over the Marines to Philippine police.

The Marines had finished counterterrorism maneuvers with Filipino troops when the alleged rape occurred at the former U.S. Subic Bay Naval base near Olongapo city, west of Manila.

The trial has stirred strong emotions in the former American colony, with a handful of anti-U.S. protesters regularly shouting slogans and carrying signs outside court.