KYIV, Sept 14 (Reuters) - Ukraine said on Thursday it attacked two Russian patrol ships and destroyed a sophisticated air defence system in the west of occupied Crimea, ramping up its strikes to challenge Moscow's dominance in the Black Sea region.

The attacks come a day after Kyiv said it seriously damaged a Russian submarine and landing ship undergoing repairs in a missile strike on a shipyard in the Crimean port of Sevastopol, home to the Russian Black Sea Fleet.

The Ukrainian military, in a post on Telegram messenger, said it hit two Russian patrol boats in the southwest of the Black Sea, causing "certain damage" in a morning attack.

"The (Sergei) Kotov was hit," military intelligence official Andriy Yusov told Reuters, sharing a grainy video circulated online by a Ukrainian government minister that appeared to show sea drones attacking a vessel at sea.

The Russian Defence Ministry confirmed an attack on the Sergei Kotov in a morning statement, but said the assault involving five sea drones was repelled. It made no mention of damage.

Reuters could not independently confirm the reports or the video.

The southwestern location of the attack would indicate Ukraine's ability to strike Russian targets far from its coast.

While Kyiv's counteroffensive in the south and east has been slowed by minefields and Russian defensive lines, fighting has escalated in the Black Sea region where Russia is imposing a de facto blockade on Ukraine's seaborne exports.

Russian drones have regularly attacked Ukrainian port infrastructure along the Danube River, a vital alternative export route for the major grain producer. It uses its Black Sea Fleet to rain down missiles on Ukrainian targets from afar.

Ukraine's embattled navy has used sea drones to strike back, hitting the Olenegorsky Gornyak landing ship near Russia's naval base at Novorossiysk early last month and a Russian fuel tanker.