A huge oil slick in the Black Sea, with the World Wildlife Fund saying at least 100 tons of oil have leaked off the city of Novorossiysk, have urged Russian scientist to sound the alarm on Wednesday.

After it emerged that the slick was much larger than initially reported, investigators launched a pollution probe and the General Prosecutor’s Office said officials were studying the coast between the resort town of Anapa and Novorossiysk.

The area is home to some of the country’s best beaches popular with Russian tourists.

A 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 (CPC), which controls the terminal, said the spill was contained, estimating that oil had spread over 200 square metres (2,150 square feet) and involved around 12 cubic metres (423 cubic feet) of oil.

By early Sunday, “the situation was back to normal” and posed no threat to either the local population or wildlife, said the consortium.

CPC’s shareholders include Russia’s Rosneft, US oil giant Chevron and Italy’s Eni.

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 had covered an area of 94 square kilometres by Sunday.

Greenpeace, which said it was asking for more information from officials, claimed the polluted area could be 400,000 bigger than initially reported.

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 — famed for its wine-making industry — and might later 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,” he told AFP.

The Space Research Institute of the Russian Academy of Sciences said it was also monitoring the pollution and citing data from satellite images said that 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 19 kilometres,” the institute said in a statement.

Victoria Glushchenko of Greenpeace Russia said in a statement 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,” she added.

Staff at a dolphinarium outside the resort town of Anapa said they had seen oil slicks on the surface and were working to protect their marine mammals.

“As soon as you put your hand in the water, the skin gets covered in a greasy film,” the dolphinarium 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 personally in charge of the case.

The general director of the consortium, Nikolai Gorban, reported to 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.

In January, jailed Kremlin critic Alexei Navalny released an investigation alleging that oligarchs had built an opulent Black Sea palace for President Vladimir Putin. Putin has denied owning the mansion, which is south of Novorossiysk.