Ukraine may have used kamikaze drones in the recent attacks on Russia's bases in Crimea, according to a military expert.

If so, it marks another way in which drones are being deployed in the Ukraine war - to hit targets behind enemy lines.

Thousands of drones are being used in the conflict by both sides to bomb targets, or to direct artillery fire onto them.

Some of the drones being used are purpose-built, military drones. However, recently Ukraine seems to be turning more towards the use of small, off-the-shelf drones.

This month, there have been attacks on a Russian military base in Saky, in western Crimea, on an airbase near Sevastopol, and on Russian ships of the Black Sea fleet in Sevastopol harbour.

Dr Marina Miron, researcher in defence studies at King's College London, says small kamikaze drones were most probably used for these attacks.

"If you look at the explosions in the attacks, they are quite small. I suspect that these are homemade kamikaze drones, which have had explosives strapped to them.

"The drones are small enough to have sneaked through Russia's air defence system."

Ukraine has neither confirmed nor denied it is behind the attacks in Crimea.

However, says Dr Miron, the attacks could show how the use of drones has changed since earlier days of the conflict, when there was more focus on attacks by military drones.

Ukraine's main military drone is the Turkish-made Bayraktar TB2. It is about the size of a small plane, has cameras on board, and can be armed with laser-guided bombs.

At the outset of the war, Ukraine had a fleet of "fewer than 50" of these, says Dr Jack Watling of the Royal United Services Institute (Rusi) think-tank.

Russia mainly uses the "smaller, more basic" Orlan-10, he says: "Russia started the war with some thousands of them, and may have a few hundred left." These drones also have cameras and can carry small bombs.

For both sides - Russia and Ukraine - drones have been most effective when used to locate enemy targets and guide artillery fire toward them.

"Russian forces can bring their guns to bear on the enemy within only 3-5 minutes of an Orlan-10 drone spotting a target," says Dr Watling. Without the drone, an attack could take 20-30 minutes to carry out, he adds.

Dr Marina Miron says drones have allowed Ukraine to stretch its limited forces.

"If you wanted to seek out enemy positions in the past, you would have had to send out special forces units... and you might lose some troops," she says. "Now, all you're risking is a drone."

In the first few weeks of the war, Ukraine's Bayraktar drones were widely praised.

"They were shown attacking targets such as ammunition dumps, and played a part in the sinking of the Moskva [warship]," Dr Miron says.

However, many Bayraktars have been destroyed by Russia's air defence systems.

"They are large, relatively slow-moving, and fly at only medium altitude - and that makes them easy to shoot down," says Dr Watling.

Military drones are expensive to replace - a single Bayraktar TB2 costs about $2m (£1.7m).

So, both sides - but especially Ukraine - are turning to small, commercial models, such as the DJI Mavic 3, which costs about £1,700.

One Ukrainian drone manufacturer estimates the country's forces have 6,000 drones, but it's impossible to confirm this.

Such commercial drones can be fitted with small bombs. However, they are mainly used to spot enemy troops and to direct attacks.

"Ukraine doesn't have as much ammunition as Russia," says Dr Miron. "Having 'eyes in the sky' to spot targets and direct artillery fire means they can make better use of what they have."

But commercial drones are much less powerful than military ones.

The DJI Mavic's total flying distance is only 30km, and it can only fly for a maximum of 46 minutes.

Cheaper and smaller drones fly for even less time, and cover shorter distances.

Russia uses radar defences against military drones, and electronic devices against commercial ones, says Dr Miron.

"Russian forces have the Stupor rifle, which shoots electromagnetic pulses," she says. This stops commercial drones from being able to navigate using GPS, she explains.

Watch: The volunteers using drones to monitor Russian troops

Russian forces have also used online systems, such as Aeroscope, to detect and interrupt communications between commercial drones and their operators.

These systems can cause a drone to crash or return to base, and can stop it sending back information.

The average Ukrainian drone barely lasts a week, according to a Rusi report.

Russia is now buying Shahid military drones from Iran, according to the White House. Houthi rebel forces in Yemen used these to attack targets in Saudi Arabia and the UAE.

The US is supplying Ukraine with some 700 Switchblade "kamikaze" military drones.

It is not known whether these were used in the recent attacks on Russian bases in Sevastopol, rather than homemade kamikaze drones.

These are packed with explosives - and are designed to loiter in the air until they find their target.

Elon Musk's SpaceX company is providing its Starlink satellite communications system to Ukraine. This creates a secure link between commercial drones and operators.

DJI has now stopped supplying drones to Russia or Ukraine.

Ukraine has launched a crowdfunding appeal to buy 200 military drones.

"As well as large drones like the [Bayraktar] TB2, they are looking for small, fixed-winged reconnaissance drones,"says Dr Watling.

Kalush Orchestra, the Ukrainian winners of the Eurovision Song Contest, sold the Eurovision trophy for $900,000 (£712,000) which it donated to the drone appeal. The proceeds will buy three Ukrainian-made PD-2 drones.