How are 'kamikaze' drones being used by Russia and Ukraine?
Ukraine has accused Russia of using Iranian drones against civilian targets in Kyiv.
rry explosives which detonate on impact and are launched in waves, making it hard to defend against.
It's believed Russia has been using Iranian-made Shahed-136 drones in the Ukraine conflict since mid-September.
Also called the Geranium-2 by Russia, it has explosives in a warhead on its nose and is designed to loiter over a target until it is instructed to attack.
Shahed-136 has a wingspan of about 2.5m (8.2ft) and can be hard to detect on radar. 
It's unclear exactly how many Russia has, but the US has said Iran planned to send hundreds to Russia. Iran has denied doing so.
Russia was first reported to have used a Shahed-136 drone on 13 September, attacking targets near Kupiansk, a city in the Kharkiv region in the east of the country.
Later that month, it was used in attacks in the south. Debris from the drones were found in wreckage in Odesa and Mykolaiv. 
In October, Shahed-136 drones landed on civilian areas in the city of Bila Tserkva, south of Kyiv.
On 14 December, Ukrainian officials say they shot down over a dozen drones during a Russian attack on Kyiv.
Russia may be using these drones rather than cruise missiles because they are relatively cheap - about $20,000 (£17,800) each. 
Ukrainian armed forces have used small arms fire, heavy machine guns, portable anti-air missiles and electronic jamming devices to try and shoot down the drones. 
Ukraine's armed forces claimed on 25 October that they had shot down 223 Shahed-136 drones in the preceding 36 days, which is 85% of the total launched by Russia.
However, it is almost impossible to shoot down every single suicide drone, when they are sent in large numbers.
"They fly low and you can send them in waves. These swarms of drones are much harder to counter by air defences," military expert Justin Crump told the BBC.
It's not clear whether these have been used by Ukraine, but the US has said it is sending it 700 of its Switchblade kamikaze drones, and experts say kamikaze drones may have been deployed in recent attacks on a Russian military base in western Crimea, on an airbase near Sevastopol, and on ships in Sevastopol harbour.
Dr Marina Miron, researcher in defence studies at King's College London, says they may have been rudimentary drones made in Ukraine.
"If you look at the explosions in the attacks, they are quite small," she says. "I suspect that these are homemade drones, which have had explosives strapped to them."
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.
US said it is sending Ukraine 700 Switchblade drones, which are also kamikaze drones. 
Russia also uses the "smaller, more basic" Orlan-10, which have cameras and can carry small bombs. 
For both sides - Russia and Ukraine - drones have been effective for locating enemy targets and guiding artillery fire toward them.
"Russian forces can bring their guns to bear on the enemy within only three to five minutes of an Orlan-10 drone spotting a target," says Dr Watling. An attack could otherwise take 20-30 minutes to carry out.
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, the Bayraktar is a large and slow moving drone, and Russia has been able to counter it effectively with its air defence systems.
Military drones are expensive to replace - a single Bayraktar TB2 costs about $2m (£1.7m).
So, both sides - but especially Ukraine - also use small, commercial models, such as the DJI Mavic 3, which costs about £1,700. 
mmercial 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 capable than military ones. 
For example, the DJI Mavic's total flying distance is only 30km, and it can only fly for a maximum of 46 minutes. 
Russia is using electronic devices to counter them, 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.
ms can cause a drone to crash or return to base, and can stop it sending back information. 