Explosions have been reported overnight near the Russian port of Novorossiysk on the Black Sea.
Footage shared on social media on Friday appeared to show loud blasts at the port, which is a major hub for Russian exports.
Russia's defence ministry said it had repelled a Ukrainian attack on its naval base there with two sea drones.
It said the drones had been spotted and destroyed by Russian ships guarding the outer harbour of the naval base.
The port temporarily suspended any movement of ships following the attack, according to the Caspian Pipeline Consortium which loads oil on to tankers at the port.
Sea drones are small, unmanned vessels which operate on or below the water's surface.
Research by BBC Verify suggests Ukraine has carried out at least 10 attacks with sea drones- targeting military ships and Russia's naval base in Sevastopol, as well as Novorossiysk harbour in a previous attack.
This is based on announcements by Russian and Ukrainian authorities, and local media reports. Ukrainian defence sources have told CNN that sea drones had also been used in an attack on the Kerch Bridge to Crimea in July.
Novorossiysk is one of the biggest ports in the Black Sea. The emergency services in the area confirmed reports of blasts and security services have been informed, Russian state media say.
Clashes in the sea have increased in recent weeks, after Russia abandoned a UN deal that enabled grain to be safely exported between Russia and Ukraine across the water.
Earlier this week, Russia attacked big Black Sea ports of Odesa and Chornomorsk, where authorities said 60,000 tonnes of grain were destroyed, as well as ports on the River Danube.
Separately on Friday, Russia also said it had downed 10 Ukrainian aerial drones over Crimea.