Ukraine has carried out a "massive" drone attack on the Black Sea Fleet in the Crimean port city of Sevastopol, damaging one warship, Russia says.
Nine drones were used, a top official said. Ukraine has not commented.
Without providing evidence, Russia accused British troops of being involved in Saturday's attack – and in blowing up gas pipelines last month.
In its response, the UK Ministry of Defence (MoD) said Russia was "peddling false claims of an epic scale".
Russia said the ships targeted in Saturday's attack were involved in the internationally-brokered deal to allow grain exports from Ukrainian ports and hours later announced that it was suspending its participation in it.
The attack comes as Ukrainian troops successfully retake territory occupied by Russian troops since they launched their invasion on 24 February.
Russia has replied by launching large-scale attacks on Ukrainian infrastructure, particularly on the country's energy grid.
Crimea was annexed by Russia from Ukraine in 2014 and is extremely symbolic for Russian President Vladimir Putin.
In recent weeks, several attacks have hit the peninsula, where the Russian army has built up a large presence.
Sevastopol is the largest city in the region and home to Russia's Black Sea fleet.
Mikhail Razvozhaev, the Russian-installed governor of the city, said Russia's navy had repelled the latest attack – the "most massive" on the city since February.
He said that all unmanned aerial vehicles (UAV) had been shot down and no "civilian infrastructure" had been damaged.
At least one vessel sustained minor damage, the Russian ministry of defence said.
"In the course of repelling a terrorist attack on the outer roadstead of Sevastopol, the use of naval weapons and naval aviation of the Black Sea Fleet destroyed four marine unmanned vehicles, three more devices were destroyed on the internal roadstead," a statement from the ministry read.
The Ukrainian government rarely comments on claims made by Russian authorities in Crimea, although senior officials often celebrate incidents in the peninsula.
Ukraine has vowed to recapture all territory that is under Russian occupation, including Crimea, which has been a key base for President Putin's forces during the invasion of Ukraine.
Without providing any evidence, the Russian defence ministry also accused British navy specialists of helping Ukrainian forces prepare the attack on Saturday morning.
It also said the same unit was "involved in plotting, organising, and implementation of the terrorist attack in the Baltic Sea on 26 September this year to blow up the Nord Stream 1 and Nord Stream 2 gas pipelines".
In its reply, the MoD said: "To detract from their disastrous handling of the illegal invasion of Ukraine, the Russian Ministry of Defence is resorting to peddling false claims of an epic scale.
"This invented story, says more about arguments going on inside the Russian Government than it does about the west."
In recent days, Russia has been engaged in what US and Ukrainian officials describe as a disinformation campaign, with unsubstantiated claims that Kyiv is preparing to use a radioactive dirty bomb, or even biological mosquitoes.
Russia has also claimed that the ships targeted on Saturday morning were involved in ensuring the "grain corridor" as part of the international initiative to export agricultural products from Ukrainian ports.
The agreement, brokered by the UN and Turkey, allowed Ukraine to resume its Black Sea grain exports, which had been blocked when Russia invaded the country. It was personally negotiated by the UN secretary general and celebrated as a major diplomatic victory that helped ease a global food crisis.
But Russia complains that its own exports are still hindered, and had previously suggested it might not renew the deal.
In recent days, Kyiv has accused Moscow of deliberately delaying the passage of ships, creating a queue of more than 170 vessels.
Russia's Black Sea Fleet was previously targeted in April this year by Ukrainian forces when its flagship, the cruiser Moskva, was sunk. The 510-crew missile cruiser had led Russia's naval assault on Ukraine, and its sinking was a major symbolic and military blow.
Earlier this month, a blast occurred on the Crimean bridge – a pivotal symbol of Russia's annexation of Crimea.
The blast killed three people, Russian investigators said.
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