The crash took place in the Tver region about 200 km (125 miles) from Moscow.

"The Vazuza is one of the sources of the Moscow city water supply, with the water going along the Moscow canal to the pumping stations," said Vyacheslav Vasilyev, head of the Emergency Ministry's Moscow accidents and environmental centre.

"And if you believe this information that 300 tonnes of fuel oil poured into the Vazuza, then this raises a huge danger for our biggest megapolis -- Moscow," he told NTV television.

It was not clear how long the oil would take to reach Moscow but any water supply problems would be a major embarrassment for city authorities after a power cut last month paralysed Russia's political and financial capital.

NTV showed viscous black oil trickling out of the capsized wagons and running in a heavy slick down to the river. Teams of workers were struggling to prevent more oil reaching the river, and bulldozers were pulling wagons back up the hill.

Officials told Russian media that as much as half the surface of the Volga -- which runs all through European Russia to the Caspian Sea -- immediately below its confluence with the Vazuza was covered in oil.

The emergencies services had deployed two boats on the river to try and break up the oil spill and halt its advance, news agencies reported.

Accidents within Russia's infrastructure are common. They are often caused, like the power cut, by poor maintenance and ageing equipment and the management of Russia's state-owned railway company blamed Wednesday's accident on repair workers.

"The rails tipped more than they should, causing the train to leave the rails. The local engineer service bears the responsibility for this," said Khasan Zabirov, deputy president of Russian Railways, in televised comments.