WASHINGTON  An American ambassador helped cover up the illegal Chinese origins of ammunition that a Pentagon contractor bought to supply Afghan security forces, according to testimony gathered by Congressional investigators.

A military attaché has told the investigators that the United States ambassador to Albania endorsed a plan by the Albanian defense minister to hide several boxes of Chinese ammunition from a visiting reporter. The ammunition was being repackaged to disguise its origins and shipped from Albania to Afghanistan by a Miami Beach arms-dealing company.

The ambassador, John L. Withers II, met with the defense minister, Fatmir Mediu, hours before a reporter for The New York Times was to visit the American contractor’s operations in Tirana, the Albanian capital, according to the testimony. The company, under an Army contract, bought the ammunition to supply Afghan security forces although American law prohibits trading in Chinese arms.

The attaché, Maj. Larry D. Harrison II of the Army, was one of the aides attending the late-night meeting, on Nov. 19, 2007. He told House investigators that Mr. Mediu asked Ambassador Withers for help, saying he was concerned that the reporter would reveal that he had been accused of profiting from selling arms. The minister said that because he had gone out of his way to help the United States, a close ally, “the U.S. owed him something,” according to Major Harrison.