Two vessels were owned by the company Volgatanker and were relatively small.

They had been carrying around 4,300 dead weight tonnes of oil each, according to Tass news agency.

A tanker used for trading Russian crude oil internationally generally has a much larger carrying capacity of around 120,000 dead weight tonnes, Bockmann said, meaning it is likely these tankers were used for transporting oil through Russia's rivers or in coastal waters.

The Kerch Strait is a key route for exports of Russian grain and it is also used for exports of crude oil, fuel oil and liquefied natural gas.

In 2007, another oil tanker - Volgoneft-139 - split in half during a storm while anchored off the Kerch Strait, spilling more than 1,000 tonnes of oil.