U.S. warships on Monday attacked two offshore Iranian oil facilities in the southern gulf in retaliation for a mine blast that ripped open the hull of an American frigate last week. The official Islamic Republic News Agency said the warships first struck the Sassan platform at 9 a.m. (1:30 a.m. EDT). Twenty-three minutes later they blasted the Nasr platform on the nearby Sirri Island. IRNA, monitored in Cyprus, noted that the ``aggressive U.S. attack'' came after the United states had blamed Iran for sowing mines in the waterway one of which blasted the hull of the USS missile frigate Samuel B. Roberts. Gulf-based Western diplomats and shipping sources said the Sassan platform was ablaze after the attack, but said they could not immediately tell how many warships participated in the attack. In Washington, The White House confirmed at a news conference early Tuesday that the U.S. warships had shelled the Iranian offshore oil platforms in retaliation for sowing mines in the gulf. The shipping executives said a number of salvage tugboats were en route to Sassan and Nasr platfores. These tugs were requesting permission from the American warships to let them rescue survivors and help extinguish fires. The Americans, the executives said, told the tugs they may rescue survivors but they may not participate in any firefighting. The two targets were pounded shortly after a warning from the U.S. warships to Iranian workers to evacuate the platform and the island. Shipping executives said their monitors picked up a radio message at 9:00 a.m. (1 a.m. EDT) from the U.S. warships to the Sassan platform telling workers to evacuate the facility, saying it ``will be shelled in five minutes.'' Sassan is sister platform to Rostam, which was wrecked by American gunners in October 1987 when the Iranians fired a Chinese-made Silkworm missile into the U.S.-flagged Kuwaiti oil tanker. Eighteen persons were injured in the tanker attack, including the captain. U.S. officials have said there was ``substantial evidence'' that Iran planted a mine that heavily damaged the hull missile frigate USS Samuel B. Roberts in the central gulf last week and injured 10 crewmen. The injured crewmen were injured and flown by helicopter to a medical facility aboard the USS Trenton. The four most seriously injured were moved to Bahrain's Salmaniya Hospital and later flown aboard a special U.S. Air Force jet to West Germany for further treatment and reunion with families. The frigate was towed to the Dubai ship repair yard on Saturday, and knowledgeable sources said it was not in danger of sinking, despite enormous damage to the hull and engine room. ``Circumstances of the attack (Monday) on Sassan were very much similar to those that preceded the shelling of Rostam last October,'' said one Dubai-based maritime salvage agent, who spoke on condition of anonymity. ``After a five-minute warning, the American guns blazed away at the platform,'' he said of the attack on Rostam. A multinational mine-sweeping effort has found six mines in the central gulf area where the Samuel B. Roberts was hit on Thursday, according to U.S. Navy sources. The mine sweeping task force included six U.S. ocean mine sweepers, four British units, two Belgium, two Dutch and four Italian. The USS Trenton, which is an amphibious transport ship, was accompanying the task force and acting as the overall command ship. U.S. defense officials suspect the mines are recently laid, Navy sources said. They said the mines looked clean and free of seaweed, indicating they had not been in the water for long. The sources said the explosives were contact mines, the same type of mines previoulsy located. The Roberts is part of the U.S. Middle East Force, which was bolstered in the gulf last summer when Kuwait registered 11 of its tankers in the United States to entitle them to U.S. Navy protection against Iranian threats. The Bridgeton, the first of the flagged tankers, struck a mine during the first U.S. escorted tanker convoy last July. The Navy then conducted at least 55 other convoys through the 550-mile length of the waterway between Kuwait and the Strait of Hormuz, outlet of the gulf, without incident. The Navy also spearheaded a massive minehunting operation with the help of its Western allies, and alone located and detonated 45 mines. The allies took care of about 20 more. The Western diplomats and shipping executives on Monday said there might be at least 10 mines in the area 55 miles northeast of Qatar, aside from the one hit by the Roberts and the two located by the Navy. The area is a major shipping lane for the U.S. convoys and is frequented by the warships and the tankers.