Prizrak (Ghost) brigade commander Alexei Mozgovoi and at least six others -- including his press secretary and armed guards -- were shot dead Saturday while travelling in an area under his control in the renegade eastern province of Lugansk.
Separatist officials said his convoy of at least two vehicles was first struck by a bomb. The survivors were then mowed down in a hail of bullets near the spot where Mozgovoi survived another assassination attempt in March.
Yet Plotnitsky tried quickly to quash talk of a potentially fatal rift emerging in the militants' loosely-organised ranks.
"The attack on Alexei Mozgovoi and his comrades-in-arms was an attack on all of us -- people who defend the right for the (separatist region) to exist," Plotnitsky told his administration's official website.
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"I mourn together with all those who knew Alexei Borisovich Mozgovoi and those who walked this journey together with him."
Mozgovoi played a central role in momentum-swinging battles that forced pro-Western President Petro Poroshenko to sue for peace on two occasions.
The former folk choir singer and poet -- believed to be around 40 -- resisted the terms of a February truce deal that permitted only limited self-rule in rebel-held lands.
Mozgovoi also featured on a US blacklist for organising a summary court system that targeted those either backing Kiev or practising Western lifestyles.
"If tomorrow I see another young woman in a cafe or bar, she will be arrested," he told reporters last year.
"A woman must be the keeper of the hearth, a mother. And what kind of mother does she become after bars?"
"A sectarian battle between the fighters is continuing on the occupied territories of Lugansk," presidential administration spokesman Oleksandr Motuzyanyk told journalists.
"The head of the Prizrak criminal gang has long been in conflict with the leadership of the so-called Lugansk People's Republic.