Russian scientists sounded the alarm on Wednesday over a huge oil slick in the Black Sea, with the World Wildlife Fund saying at least 100 tonnes of oil have leaked near the city of Novorossiysk.
Investigators launched a pollution inquiry after it emerged that the slick was much larger than initially reported.
The General Prosecutor's Office said officials were studying the coast between the resort town of Anapa and Novorossiysk.
The area has some of Russia's best beaches, which are popular with domestic tourists.
The leak occurred at a sea terminal near the southern port city at the weekend as the Greek-flagged Minerva Symphony was loading oil.
On Monday, the Caspian Pipeline Consortium, which controls the terminal, said the spill was contained, estimating that about 12 cubic metres of oil had spread over 200 square metres.
By early Sunday, "the situation was back to normal" and posed no threat to the local population or wildlife, the consortium said.
CPC's shareholders include Russia's Rosneft, US oil giant Chevron and Italy's Eni.
But the WWF and Russian scientists said the oil slick was much more serious than initially reported and could harm the environment.
The conservation group said it had launched its own surveillance and found the slick covered an area of 94 square kilometres by Sunday.
Greenpeace said it was asking for more information from officials.
The WWF estimated that at least 100 tonnes of oil, "and most likely even more", had been released into the Black Sea.
"Despite the prompt involvement of rescue teams, the oil spread over a colossal area," the WWF said on Facebook, adding that marine wildlife could be affected.
Aleksei Knizhnikov, head of the responsible industry programme at WWF Russia, said the slick was drifting north, having already reached Abrau-Dyurso, which is famous for its wines, and might reach the Utrish Nature Reserve.
"We can say that there is no objective information about the scale of the spill on the part of regulatory authorities," Mr Knizhnikov told AFP.
The Space Research Institute of the Russian Academy of Sciences said it was also monitoring the pollution and, quoting data from satellite images, said the oil had spread over an area of nearly 80 square kilometres.
"On August 8, the oil slick spread from the shore into the open sea over a distance of 19km," the institute said.
Victoria Glushchenko of Greenpeace Russia said that if the estimates of the space research institute were correct: "This spill will threaten fish, birds and marine ecosystems in the area."
"In addition, the health of people, including tourists who will find themselves in the pollution zone, is at risk."
Staff at a dolphin aquarium outside Anapa said they had seen oil slicks on the surface and were working to protect their creatures.
"As soon as you put your hand in the water, the skin gets covered in a greasy film," the reserve said on Instagram.
On Wednesday afternoon, Russian authorities said they were looking into the situation.
The head of the environment watchdog Rosprirodnadzor, Svetlana Radionova, said she was in charge of the case.
The general director of the consortium, Nikolai Gorban, reported to Russian Energy Minister Nikolai Shulginov about the spill clean-up and containment work.
"The minister stressed the need to verify information on the volume of the oil spill," the Energy Ministry said.