A Russian naval ship has been damaged in a Ukrainian naval drone attack in the Black Sea, Ukrainian sources say.
The assault reportedly occurred near the Russian port of Novorossiysk, which is a major hub for Russian exports.
Russia's defence ministry said it had repelled a Ukrainian attack on its naval base there which involved two sea drones, but did not admit any damage.
But Ukrainian security service sources say the Olenegorsky Gornyak was hit and suffered a serious breach.
They told the BBC a sea drone was carrying 450kg (992lb) of dynamite when it hit the ship.
Russia made no mention of any damage in its report of the incident.
Sea drones are small, unmanned vessels which operate on or below the water's surface.
A video sent to the BBC by a source with Ukraine's security service appears to show the drone approaching a ship thought to be the Olenegorsky Gornyak.
The footage shows a vessel travelling right up to the side of a ship before the feed cuts out, apparently on impact.
Another unverified video is thought to show the ship listing to one side.
The Olenegorsky Gornyak is a landing ship, designed to launch amphibious forces close to shore for beach landings but also to dock and quickly unload cargo at ports.
Any damage to it may interfere with Russia's efforts to resupply forces fighting in occupied southern Ukraine, although the Russian fleet is unlikely to be significantly impacted.
The Novorossiysk port temporarily suspended any movement of ships following the assault, according to the Caspian Pipeline Consortium, which loads oil on to tankers at the port.
Research by BBC Verify suggests Ukraine has carried out at least 11 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 in Crimea in July.
Friday's attack comes just a few days after Ukraine revealed the external appearance and some details of what they have described as their "new" weapon - unmanned naval drones.
According to BBC Russian military correspondent, Pavel Aksenov, Ukraine has been using these drones to attack Russian ships since last year so they are not exactly new.
However, he says these vessels represent a new stage in the evolution of naval warfare, where small, unmanned boats can inflict damage on large ships with powerful weaponry.
This is not the first time Ukraine has tried to hit the Novorossiysk port, and the reasons are obvious.
Around 1.8m barrels of oil are exported from there every day - around 2% of the global supply.
It is also an important naval base for Moscow.
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.
Ukrainian ports have been pummelled by Russian drones and Kyiv seems to have been keen to respond.
It is also more willing to admit to strikes involving sea drones than the attacks seen further inside Russia.
President Zelensky has warned of the war "coming to Russia", despite suggesting a peace summit could happen "as early as the autumn".
Neither side appears overly keen to set the conditions for that.
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.
Source: BBC