The coast guard has taken samples of fuel from at least 24 ships that have sailed across the Saronic Gulf in recent days as part of its efforts to track down the vessel that caused large oil slicks to appear on Saturday at two beaches in southern Athens, officials said yesterday. The samples have been sent to the state laboratory for tests. Officials hope they will then be able to match the samples with the substance that was collected at the beaches in Vouliagmeni and Varkiza. Coast guard sources said they believe that the oil slicks, which were 20-30 meters wide, were caused by a leak from a large vessel as fuel was moved from one tank into another. The Greek coast guard has also asked for help from its foreign counterparts, as a number of ships that were in the area in the days leading up to the spill have since left Greek waters. Special teams were still cleaning up the slicks yesterday but dermatologists said that swimmers who came into contact with the oil slick have nothing to worry about as long as they washed themselves properly afterward. Although it is unusual for oil slicks to appear at popular beaches like the ones in southern Athens, the coast guard said yesterday that it was familiar with the practice of cleaning up the mess left by some ships. Officials said that there were 782 cases of pollution of the sea between 2003 and 2005.