(CNN) -- Rust-colored oil washed ashore on barrier islands off Alabama and Mississippi on Tuesday, while more patches of crude offshore appeared to be moving toward those states' coasts, authorities reported.
Researchers scrambled to clean up tar balls and puddles of oil from the beaches of Alabama's Dauphin Island, while a strip of oil about two miles long and three feet wide stretched along Petit Bois Island, about five miles away off Mississippi, Gov. Haley Barbour's office reported.
It marked the first time oil has hit Mississippi's shores since the largest oil spill in U.S. history erupted in late April. And while tar balls associated with the Gulf spill had hit Dauphin Island, about 35 miles south of Mobile, in early May, residents said that Tuesday was the first time they had seen oil hitting the beach.
Only part of the island's beaches have been lined with protective booms, with much of those barriers lined up near a protected wildlife area on the west end of the island.
Annette Engel, a Louisiana State University researcher on Dauphin Island, said the oil is believed to be from BP's ruptured well off Louisiana. She predicted much more would be hitting the coast in coming days -- but vacationers remained on the beach, and some were still swimming in the blue-green waters as the cleanup continued.
And researchers from the Dauphin Island Sea Lab spotted large patches of the reddish-brown "weathered" oil during a water-sampling expedition offshore Tuesday, said John Dindo, the laboratory's associate director. Dindo said the oil spots on the water ranged from the size of a half-dollar coin to 30 to 40 feet.
A half-dozen boats could be seen skimming oil off the surface about 13 miles south of the island, he said. However, "They were covering a very, very small spot in the ocean compared to what we saw," he said.
The National Oceanographic and Atmospheric Administration had warned earlier this week that the spreading slick from an undersea BP oil well was heading toward the Alabama and Mississippi coasts. Dindo said tides in the area are running east and winds have been out of the southwest, driving the oil toward beach towns on the eastern side of Mobile Bay.
Coast Guard Adm. Thad Allen said earlier Tuesday that authorities were investigating reports that the outer sheen of oil was reaching coastal waters off Mississippi and Alabama, but said those reports had not been confirmed.
"We've dispatched survey teams to see what the impact is out there, and to the extent that is required, we'll move resources that direction," Allen, the federal government's national incident commander, told reporters in New Orleans, Louisiana.
CNN's Patrick Oppmann and Matt Smith contributed to this report.