They say their "Death" unit fighting Ukrainian forces has 300 people, mostly former state security troops in the mainly-Muslim region where Moscow waged two wars against Islamic insurgents and which is now run by a Kremlin-backed strongman.
Seasoned Chechen fighters, whose combat experience often dates back to the 1994-96 and 1999-2000 wars, fight on both sides in east Ukraine, adding to the complexity of a conflict in which the West says Russian troops are involved.
"This is volunteer battalion Death," deputy commander of the group who only gave his nickname "Stinger", told Reuters in a former tourist camp the unit turned into their base outside of the rebel stronghold of Donetsk in east Ukraine.
"There are about three hundred of us in Donetsk region. We have battlefield experience from 10 to 20 years starting from 1995," said the man, in his forties, a pistol fixed to his thigh.
He had a little Chechen flag in green, white and red stitched to his cap and speaks Russian with a strong Caucasus accent.
Several cars with Chechen registration plates are parked in the camp.
Russia sides with the rebels in east Ukraine but denies sending serving troops to reinforce them.
Some fighters on the ground admit to being former Russian servicemen, or "on leave".
Moscow has said any Russians fighting there are volunteers.
In Chechnya, two brutal wars quashed the separatist insurgents but unrest is still simmering.
Gunmen attacked a police post and captured a building in the regional capital of Grozny last week and at least 20 people, including ten police and ten suspected militants, were killed in gunbattles that ensued.
Violence erupted just hours before Russian President Vladimir Putin had been due to give a major speech in Moscow, a symbolic challenge to the man who came to power 15 years ago credited for the Russian army victory in the second Chechen war.
Re-establishing Moscow's control over Chechnya and then introducing an uneasy peace under Ramzan Kadyrov, whom critics and rights campaigners accuse of heavy-handed tactics and massive rights violations, is seen by Putin's supporters as a key achievement.
In Ukraine, Stinger's men are sworn enemies with another group of Chechens who fight on the opposite side of the conflict and support the Kiev government troops.
Some of them have Western passports after fleeing Russia following the two wars.
They say Moscow is theirs and Kiev's joint enemy and that Chechnya is occupied by Russia.
Stinger, however, said Chechnya was being destroyed in the wars of 1990s and became peaceful again only when some local leaders allied with the Kremlin.
Some of those in the "Death" unit said they had initially fought against Russia in Chechnya but later switched sides and were amnestied by a former Kremlin-allied head of the region, Ramzan's father, Akhmet Kadyrov.
"Now we are (former) soldiers and officers of the Russian army, of Russian special forces, mostly veterans of war campaigns," Stinger said.