A Russian Black Sea city declared a state of emergency Thursday after a burst pipeline spewed oil into the landlocked water body, with stormy weather hampering cleanup efforts.

The pipeline near the city of Tuapse burst late Tuesday, according to ChernomorTransneft, a subsidiary of Russia's main oil transport company Transneft.

"The wall of the pipeline broke due to... a landslide," the company said in a statement, adding that the rupture caused 8.4 cubic metres to leak out into the Tuapse river, which empties into the Black Sea.

Environmentalists warned however that the volume of the spill could be nearly 100 times greater than claimed by Transneft.

The oil transport company said the damaged section of the pipeline -- about nine kilometres (five miles) from the Black Sea coast -- was under construction by a subsidiary of oil giant Rosneft and was not yet in use by Transneft.

Rosneft also operates a major oil refinery in Tuapse.

Russia's sea and river transport agency said a cleanup mission was launched on Wednesday afternoon, though stormy weather precluded the use of boats.

By Thursday, the local authorities declared a state of emergency in Tuapse and more than 300 workers were at the scene, according to the Krasnodar regional government website.

"There is a state of emergency for Tuapse city," a statement on the Krasnodar regional government website said. "Work is complicated by a storm, with waves two to three metres (up to 10 feet) high," it said.

World Wildlife Fund said Thursday that the spill already polluted 15 kilometres of the Black Sea shore, and accused Rosneft and Transneft of failing to act quickly and understating the real extent of the damage.

"According to WWF's information regarding the surface area and characteristics of the spill, the volume of the spill could be 500 to 700 tonnes (nearly 800 cubic metres)," WWF said Thursday.

The organisation said the consequences could have been avoided if the energy company alerted local authorities about the accident immediately instead of delaying its response for many hours.

Tuapse borders the resort city of Sochi, where Russia hosted the Winter Olympic Games in February.