Ukraine has accused Russia of ramming one of its tugboats off the coast of Crimea in "openly aggressive actions".

The Ukrainian navy said a Russian coastguard vessel, named the Don, damaged the boat's engine, hull and side railing.

The incident allegedly took place as three Ukrainian navy boats - including two small warships - headed for the port of Mariupol in the Sea of Azov, an area of heightened tensions between the countries.

Russia - which claims the waters off Crimea after annexing the peninsula in 2014 - accused Ukraine of illegally entering the area and deliberately provoking a conflict.

It placed a huge cargo ship beneath the Russian-controlled Kerch Strait Bridge to block the Ukrainian vessels.


Image: The Ukrainian navy issued an image allegedly showing a Russian border guard trying to stop its tugboat

"Their goal is clear - to create a conflict situation in the region," Russia's Federal Security Service (FSB) said in a statement.

But the Ukrainian navy said Russia had been informed in advance about the planned journey.

"Russian coastguard vessels… carried out openly aggressive actions against Ukrainian navy ships," its statement said.

Russia's Interfax news agency reported later that Moscow has since closed the passage to civilian traffic due to heightened security concerns.

A 2003 treaty designates the Kerch Strait and Sea of Azov as shared territorial waters but Russia has been asserting greater control over the area since 2015.

In September, the Ukrainian navy accused Russian border guards of "acts of provocation" against its ships taking the same route.

Ukraine has increased the number of navy ships and border guard patrols in the Sea of Azov, which is reached via the Kerch Strait between Crimea and Russia.

Kiev says its naval build-up is due to Russia stepping up controls in the area this year and carrying out lengthy checks on commercial shipping.

Mariupol is close to the region of eastern Ukraine controlled by Russian-backed separatists in a conflict that has caused at least 10,000 deaths since 2014.