Iran charged that Iraq has again resorted to the use of chemical weapons in their 5 1/2-year war, after suffering 4,000 casualties in a major weekend battle in the Iranian-occupied Faw Peninsula. The Iraqis said the report was fabricated. The fighting raged between the oil port of Al Faw and an Iraqi naval base at Umm Qasr, near Kuwait. Last month, a U.N. investigation team visited the area and confirmed that Iraq had deployed chemical weapons. A U.S. report reached similar conclusions.