One of the vessels broke apart and sank, while the other ran aground. One person from the crew is reported to have died and 26 were rescued.
The Russian state-controlled news agency Tass had previously reported that the vessels had a combined cargo of 9,200 tons of oil.
Now, Russian authorities are announcing that oil has leaked out and that work will begin to catch it. Russia does not disclose the size of the oil spill, but describes it as partial.
Ukrainian authorities, on the other hand, have called the leak extensive and say it risks causing a major environmental disaster in the Black Sea.
The Kerch Strait is located between the annexed Crimean Peninsula and Russia. The strait separates the northern Black Sea from the Sea of Azov.
Since the EU, among others, has imposed a range of sanctions against Russia, the country has used a so-called shadow fleet of old tankers to transport oil to various places around the world.
