A Ukrainian sea drone attack on Russia's Crimean bridge in July had "overturned" naval operations and forced Moscow to resort to ferries to move weaponry, the head of Ukraine's main intelligence agency said in a video broadcast on Friday.

Vasyl Maliuk, head of the Security Service of Ukraine (SBU), said the second of two major attacks in August had seriously disrupted operations on the 19-km (12-mile) bridge, Europe's longest, and dented the notion of Russian invincibility.

"We have practically overturned the philosophy of naval operations," Maliuk said in the first of a series of televised documentaries entitled "SBU, the Special Operations of Victory."

"We have destroyed the myth of Russian invincibility. The country is a fake. The bridge is doomed. Plenty of surprises lie ahead and not just the Crimean bridge."

Maliuk outlined how the attack, endorsed by President Volodymyr Zelenskiy, involved five "Sea Baby" seaborne drones -- remotely controlled from Kyiv, 1000 km (600 miles) to the north.

Video clips showed a fiery explosion on the bridge and technicians cheering the outcome in a control room.

The documentary said reports submitted by agents showed that six of eight supporting structures were destroyed and two damaged and Russian forces switched to ferries to supply their troops with weapons.

Russia said the July attack killed two people on the bridge linking the Crimean peninsula with the Russian mainland by road and rail. Traffic has since been operating on the bridge, though Russian officials say repair work is still proceeding.

The bridge was completed amid great fanfare in 2018, four years after Moscow annexed the peninsula, proclaiming it Russian territory forever.

The attack on the bridge is one of a number of Ukrainian offensive actions in the Black Sea, including a missile assault on the Black Sea Fleet headquarters in Sevastopol in September.

Zelenskiy said this month that Ukraine has seized the initiative from Russia in the Black Sea and, thanks to the use of naval drones, forced Russia's naval fleet and warships to pull back.