U.S. FORCES DESTROYED OIL RIG, RAIDED ANOTHER U.S. warships destroyed an Iranian oil platform in the Gulf on Monday and the Navy also raided a second oil rig in retaliation for Friday's Iranian missile attack on a U.S. flag tanker, the Pentagon said. President Reagan and Defense Secretary Caspar Weinberger warned Iran of even stronger countermeasures if the military escalation between the two countries continued in the volatile waterway. Reagan said he ordered the strike on the first platform by four U.S. destroyers, which fired about 1,000 five-inch shells at the Rostam oil rig 120 miles east of Bahrain in the central Gulf. A short time later, Navy personnel boarded a second Iranian oil platform about five miles to the north and destroyed radar and communications equipment before leaving the structure, the Pentagon said. Defense Department spokesman Fred Hoffman told reporters that the second platform had been abandoned by Iranian personnel during the shelling and destruction of the first rig. The United States said both platforms were being used to keep track of Gulf merchant shipping and to launch speedboat attacks on such shipping by Iranian Revolutionary guards.