The so-called "Kadyrovtsy" -- the Kadyrovites -- are Chechen militias with a sinister reputation deployed alongside Russian troops in Ukraine.

Ukrainians have said that the Chechens have been among the most brutal of the Kremlin's invading forces.

Kadyrov -- a Putin ally who rules Chechnya and is accused of rights abuses including torture and executions -- proudly posts videos of his men fighting in Ukraine on Telegram.

He alleges they are fighting the "Nazis of Kyiv", using the Kremlin's language.

The son of a Chechen independence leader who switched sides to join the Russians, Ramzan Kadyrov is President Vladimir Putin's protege and is regularly accused of shocking human rights violations in the Muslim-majority republic of Chechnya.

Kadyrov welcomed Putin's invasion of Ukraine and immediately said he would send forces there.

The 45-year-old bearded autocrat claimed last month that he had personally travelled to Mariupol, Ukraine's besieged port that has suffered the worst devastation since the Kremlin launched its attack.

He posted a photo of himself posing with around 30 men, saying it was in Mariupol.

He also claimed he had found and "punished" with his own hands a Ukrainian soldier who had tortured a Russian.

His men regularly boast of proudly having "taken" wounded Ukrainian soldiers.

Such brutality lives up to the reputation the Kadyrovtsy earned in the Kremlin's previous conflicts -- from Chechnya to Ukraine in 2014 and Syria.

Aurelie Campana, an expert in political violence and Russia at the University of Laval in Canada, said the deployment of the Kadyrovtsy is part of Moscow's "psychological war" on Ukraine.

"The announcement of the entry into the war of Kadyrov's troops and the propaganda that surrounds it are part of this effort to destabilise the enemy," she said.

Breeding fear

"They are known for their cruelty," Campana wrote in an analysis on The Conversation website.

"Involving Chechen troops serves to fuel fear within the Ukrainian population."

At the start of the war, rumours spread that Putin -- counting on a quick overthrow of Ukrainian President Volodymyr Zelensky had sent the Chechens to kill him.

Kadyrov had also vowed that Zelensky would soon be "the former president of Ukraine".

Zelensky then became the symbol of the resistance of an entire nation, defying Putin with daily social media appearances that are popular globally, in which he often mocks the Kadyrovtsy.

How many of the Chechen forces are in Ukraine?

Kadyrov himself said in mid-March around 1,000 of his men were there. There is no way to verify the figure.

"Nobody knows exactly how many Chechens are fighting in Ukraine or where exactly they are deployed," Russian political expert Alexei Malashenko told AFP.

Other Chechens, those who oppose Kadyrov, have joined Ukrainian forces to fight the Kremlin.

Experts say the Kadyrovtsy have been sent by the Kremlin to maintain order.

While their brutality is unquestionable, their success are yet to be proven.

Kadyrov had triumphantly announced that his men had captured Mariupol's city hall, before publishing a video that showed he was in fact referring to a different administrative building.

"Kadyrov is taking part in the Ukraine operation to show his total loyalty to Putin and to keep his influence," political expert Konstantin Kalachev said.

"For him, the operation is personal publicity."

'Disciplining the Russians'

The Chechen leader is himself suspected of being behind several high-profile assassinations in Russia -- including of Kremlin critic Boris Nemtsov and journalist Anna Polikovskaya.

His commander in Mariupol Ruslan Geremeyev is believed to have organised the gunning down of Boris Nemtsov right by the Kremlin walls in 2015.

Geremeyev was injured in late March while fighting Ukrainian forces in Mariupol.

In Ukraine, the Kadyrovtsy could also serve as a force to discipline Russian soldiers as they had done with pro-Moscow separatists in the 2014 war, some commentators say.

"The experience of Kadyrov's troops could not only be an asset to overcome local Ukrainian resistance, but also for disciplining Russian troops and their cronies," Campana wrote.

Indeed, Kadyrov's men have few friends in the Russian army, where resentment left over from bloody wars in Chechnya in the 1990s still runs high.

"But Putin trusts them completely," Malashenko said.

"For Kadyrov, participating in the Ukraine operation is a personal success."