Weathered oil in the form of tar has washed up on some Louisiana beaches from Gulf waters churned by Hurricane Isaac, prompting restrictions of fishing in some waters and tests to determine whether the source is submerged oil from the 2010 Deepwater Horizon disaster.

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"I'd say there's a smoking gun," said Garret Graves, Gov. Bobby Jindal's top adviser on coastal issues. He said tests were being done to verify the source of the oil.

"It's an area that experienced heavy oiling during the oil spill," he said.

Officials Tuesday evening restricted fishing in waters extending a mile off a roughly 13-mile stretch of coastline from Port Fourchon eastward to just west of Caminada Pass.

Recreational rod and reel fishing can continue but commercial and recreational shrimping, crabbing and commercial fin fishing was prohibited there.

The state Wildlife and Fisheries Department said there was a large mat of tar on one beach and concentrations of tar balls on adjacent beaches. Graves said later surveys found several more mats. The size of the tar mats was not immediately clear. Graves said high water has prevented a thorough examination.

"With many of the southern parishes of Louisiana still inundated with flood waters and not accessible at this time, it is premature to make any claims about possible oiling there whether it is from the Deepwater Horizon accident or any other source," BP PLC said in a statement emailed late Tuesday.

It said BP was awaiting test results from an area west of Grand Isle.

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Isaac leaves oil tar on beaches