Fears two oil slicks would pollute eastern Corsica's holiday beaches eased Saturday after French officials prepared for the worst and naval boats armed with clean-up equipment arrived off the Mediterranean island.

"We are more reassured now because the pollution is drifting away from the coast," maritime prefecture spokesperson Christine Ribbe told AFP.

"The pollution is breaking up and now about 10 kilometres (six miles) or so offshore. But we have to remain very careful because the situation can change with the currents."

The authorities had voiced concern the oil would pollute the coast from Aleria to Ventiseri on Saturday and closed beaches along the 40-kilometre stretch and banned fishing.

Two naval ships, equipped with "anti-pollution material and specialised staff" were already picking up oil from the surface of the sea.

Some 80 members of the security forces and rescue services were also being drafted in to aid with any clean-up.

Two French navy ships as well as some 80 members of the security and rescue forces have been drafted in to deal with the oil spill
Two French navy ships as well as some 80 members of the security and rescue forces have been drafted in to deal with the oil spill Marine Nationale / -

The heavy-grade oil, which local authorities said appears to have come from a ship cleaning out fuel tanks, was first detected around midday Friday during an aerial surveillance operation.

By Saturday, officials had detected two large slicks stretching over 19 nautical miles (35 kilometres), one 800 metres offshore, the other 3.5 kilometres.

Francis Giudici, mayor of Ghisonaccia, where the beach was closed, told AFP: "We are hoping we'll avoid the pollution, but it will be complicated.

"There's also been a lot of anger," he said. "We really don't need this at the start of the (holiday) season."

France's Minister for the Sea Annick Girardin told reporters: "We have come here determined to find those who" caused the oil spill.

"They are thugs and should be treated as thugs."

Prosecutor Dominique Laurens said France's maritime gendarmerie had opened an investigation and the polluting vessel would be identified.