Iraqi warplanes attacked an Iranian supertanker in the Persian Gulf in their third air raid on vessels off Iran in less than a week, shipping officials said. They said the 259,000-ton Cypriot-flag Anax was hit 150 miles south of Iran's Kharg Island oil-loading terminal. The vessel suffered minor damage. Tehran's official news agency said that Iranian ground batteries shot down an Iraqi Mirage warplane over the northern sector of the front. The battle reports came amid indications that Iran is continuing to prepare for a new ground offensive against Iraq.