Two attacks by Ukrainian drone ships against Russian warships have been thwarted, according to Moscow.

While aerial drones have been used to great effect by Ukraine and Russia since the start of the war, a new form of drone technology has also been gaining momentum in the Black Sea.

Drone ships are small, unmanned vessels. Unlike aerial drones, these ones operate on or below the water’s surface.

They come in all shapes and sizes and are used for a variety of tasks, including environmental monitoring.

These drones can be used for military purposes too – like clearing mines, carrying out surveillance or detonating near targets like enemy ships.

Many drone ships have been pictured since the start of the war, including one which reportedly washed up on the Crimea shores.

As well as drones that operate on the water’s surface, Ukraine recently unveiled a prototype of an unmanned, undersea drone called the Toloka TLK-150.

Common drone ship features include built-in explosives and cameras which beam back images to the person controlling it.

Long-range targets are typically pre-programmed into the drones when launched.

They are then guided remotely by a human as it closes in on the target, explains Sidharth Kaushal from the defence think tank Rusi.

Some Ukrainian drone ships have been developed with help from crowdfunding campaigns. They are usually made with “off-the-shelf” components normally for commercial – rather than military – use, adds Mr Kaushal.

Russian media and bloggers have claimed that Russia also has drone ships. Some claim they were used in an attack on a bridge in the port city of Odesa. However, the BBC has seen no evidence to support this.

It’s unknown how many drones ships each side holds. It’s also unclear how much they cost, but one drone publicised by the Ukrainian government has a price tag of $250,000 (£197,000).

That would be cheaper than many types of long-range missile. Drone ships can be also deployed quickly and without a fully-trained crew.

“Even if you intercept a lot of low-cost objects, all you need is one to get through and damage a much more expensive asset and the model has paid for itself,” says Mr Kaushal.

BBC Verify research suggests Ukraine has carried out at least 10 attacks with drone ships – targeting military ships, Russia’s naval base in Sevastopol, and Novorossiysk harbour. This is based on announcements by Russian and Ukrainian authorities, and local media reports.

Ukrainian defence sources have told CNN that sea drones were also used in an attack on the Kerch Bridge in July.

Some attacks have been far from Ukrainian shores.

In May, footage emerged showing drone ships approaching a Russian intelligence-gathering ship called the Ivan Khurs, but it’s unclear if the ship was damaged.

Russia says the incident took place 90 miles (140km) north of Turkey’s Bosphorus Strait – around 120 miles (193km) from the Ukrainian coast. This suggests these drones can potentially travel long distances.

Ukraine’s deployment of drone ships, at relatively low cost, marks a new era for naval warfare. The tactic poses an increasing risk to Russia, according to some analysts.

Compared with naval vessels, drone ships are harder to detect on radar because they travel low on the water and they make far less noise.

The attack on Russia’s naval base in Sevastopol, back in October 2022, was the first in recorded history to use both ship and aerial drones.

At least three Russian ships were damaged in the attack, according GeoConfirmed, an organisation that analyses satellite and other open source imagery. Since then, Russia has significantly bolstered defences around the base according to recent satellite images seen by BBC Verify.

While Ukraine does not have a substantial navy, its surface drones have stopped Russia from taking full control of the Black Sea, says Katarzyna Zysk, a professor at the Norwegian Institute of Defence Studies.

Despite this, drone ships have some disadvantages.

On-board sensors may have a narrow field of view, which will make it hard to track moving targets without accurate location data or to spot camouflaged ships.

Those with onboard cameras also need constant communication with their controller to direct them to a target, so any issues with streaming video will jeopardise their mission.

“It’s not a revolution as yet,” says Prof Zysk. “We’re still in an experimental phase”.

Nonetheless, Ukraine’s strategy has caught international attention, and is “pushing other navies to develop these kind of systems and their operational practice,” she adds.