Two ageing tankers were hit by storms in mid-December in the Kerch Strait between Crimea and southern Russia, with one breaking apart and sinking and the other running aground.

"At this moment the fuel oil has been detected on the Azov shore," Yevgeny Balitsky, the Russian-installed head of the occupied part of the Zaporizhzhia region, said on Telegram.

The official said an oil slick more than 14 kilometres (9 miles) long containing "tiny fragments of solid fractions of oil", had been discovered along the Berdyansk Spit, a long strip of land projecting into the Sea of Azov off the port city of Berdyansk.

Another much smaller slick was found by another spit further east, he added.

This week President Vladimir Putin criticised the efforts to clean up the oil, which has spread east to southern Russian beaches and as far west as the Crimean city of Sevastopol.

Putin called the cleanup "obviously not enough", after the spill contaminated thousands of tonnes of sand and soil. It also killed sea birds and mammals.

Russia on Friday said that additional oil was leaking from the tanker that ran aground close the shore. The two ships were carrying more than 9,000 tonnes of fuel oil known as mazut.

Mazut is a heavy fuel oil that is difficult to clean up as it does not float on the surface.