Mikhail Filiponenko, a member of the regional assembly in the Russian-occupied Luhansk region of eastern Ukraine, was killed in a car bombing, his son said on November 8.
No further details were immediately available.
Ukraine's Main Directorate of Intelligence claimed responsibility for what it called the "liquidation of a war criminal."
"A special operation to eliminate the executioner Filiponenko was implemented together with representatives of the resistance movement," the directorate said in a statement on Telegram, adding that "hunting continues" and "all war criminals and collaborators will get a just retaliation."
Filiponenko was a former head of Moscow-backed separatist troops in Luhansk and had represented the Luhansk separatists at the cease-fire monitoring center in the region before Russia's full-scale invasion of Ukraine last year.
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He reportedly survived another assassination attempt on February 21 last year, three days before the invasion.
Kremlin-backed separatist leaders of parts of Ukraine's Donetsk and Luhansk at the time asked Russian President Vladimir to recognize the two regions' independence from Ukraine.
Russian lawmakers in mid-February 2022 also called on Putin to recognize separatist-controlled parts of Ukraine's eastern regions of Luhansk and Donetsk as independent states and ratified corresponding documents after Putin signed them.
Moscow used the documents to justify its unprovoked invasion of Ukraine, launched on February 24, 2022.
Before 2010, Filiponenko was a lawmaker in Luhansk's Lenin district, representing Ukraine's now banned pro-Russian Party of Regions.
After Russia unilaterally declared its annexation of Ukraine's Luhansk, Donetsk, Kherson, and Zaporizhzhya regions in September 2022, Filiponenko became the leader of Russia's ultranationalist Liberal Democratic Party's lawmakers in the Moscow-installed regional parliament in Luhansk.
Ukrainian authorities launched a probe against Filiponenko and added him to the wanted list on charge of violating Ukraine's territorial integrity.
Several Ukrainian nationals who collaborated with the occupying Russian authorities have been targeted by unknown attackers in Russian-held territories in recent years. Some of them were killed.
Russia has accused the Ukrainian special services of organizing the assassinations. Kyiv either does not comment or denies any involvement in the attacks.