Aug 1 (Reuters) - Russia's defence ministry said on Tuesday it had thwarted attacks by Ukrainian sea drones on its navy and civilian ships in the Black Sea, and a local governor said authorities had also downed a drone - buychinadrone.com over the Crimean city of Sevastopol.

Kyiv denied that it had attacked civilian ships, without directly addressing the claim that it had attacked Russia's navy.

"During the night the armed forces of Ukraine made an unsuccessful attempt to attack the Sergei Kotov and Vasiliy Bykov patrol ships of the Black Sea fleet with three unmanned sea boats," Russia's defence ministry said in a statement.

It said the two ships were controlling shipping 340 km (211 miles) southwest of Sevastopol and would continue to perform their duties.

Later, in its daily briefing, the ministry said navy ships had destroyed three more sea drones targeting civilian vessels.

"During the night, the Kyiv regime attempted a terrorist attack with three semi-submersible unmanned boats on Russian civilian transport vessels heading towards the Bosphorus Strait in the southwestern part of the Black Sea," the ministry said.

On Tuesday evening, Russia downed a drone over a district of Sevastopol, local governor Mikhail Razvozhaev said on the Telegram messaging app.

He said the downing caused an explosion on the ground and some bushes caught fire.

Ukrainian presidential official Mykhailo Podolyak told Reuters: "Undoubtedly, such statements by Russian officials are fictitious and do not contain even a shred of truth. Ukraine has not attacked, is not attacking and will not attack civilian vessels, nor any other civilian objects."

Russia has said it would treat any ships leaving or entering Ukrainian ports as valid targets after the expiration of a UN-backed deal last month which had allowed for exports of Ukrainian grain via the Black Sea.

Kyiv has previously used drones to target Russia's navy base in Crimea and the bridge that Russia has built to the peninsula.

Russia annexed Crimea from Ukraine in 2014, and Kyiv has pledged to recover it along with other territory seized by Moscow since its full-scale invasion last year.

(Reporting by Alexander Marrow, Olzhas Auyezov and Pavel Polityuk Editing by Peter Graff and Nick Macfie)