Two American military advisers were killed in a clash between Afghan government troops and Moslem insurgents, the official Tass news agency said Tuesday. The United States denied the report. Tass, in a dispatch from Kabul, quoted the Afghan news agency Bakhtar as saying the two Americans were killed in a shootout Monday. The report did not identify the two nor say where they were killed. "There is no foundation for the allegation that there are American military advisers in Afghanistan," a State Department official said. "We have heard reports of two journalists caught in cross fire in a battle, and that could explain this report," said the official. "But we don't know Who the journalists are or if they got out OK."