Olexiy Kovalev, a former Ukrainian MP who collaborated with occupying forces, has been shot dead in his home in the southern Kherson region, Russian media reported.
Kovalev, 33, died after receiving a gunshot wound to the head on Sunday, media outlets such as Baza claimed.
His girlfriend suffered stab wounds in the attack and died in hospital on Monday.
“This morning, Olexiy Kovalev’s girlfriend died in the intensive care unit after being wounded with a shiv,” said Sergei Khlan, adviser to the head of Kherson’s military administration.
Kovalev had previously served in President Volodymyr Zelenskyy’s party and as a deputy head of agriculture in Kherson.
His body was reportedly found by his mother after the attack on Sunday.
He was reported in June to have publicly supported Russia’s occupation of the Kherson region.
Olexander Scherba, a former Ukrainian ambassador to Austria, posted a photo of Kovalev on Twitter and asked “was it worth it?”.
“Yet another traitor got eliminated in the occupied Kherson,” he wrote.
Russian forces seized Kherson in early March.
It was the first major city to fall after the invasion of Ukraine on February 24.
By mid-March, invading troops had taken control of the entire Kherson region, which is important for Ukrainian agriculture and borders the Crimean peninsula, which Russia annexed in 2014.
Ukrainian forces have started a counter-offensive to retake the strategic territory, Mr Khlan said on Monday.
"Today there was a powerful artillery attack on enemy positions in ... the entire territory of the occupied Kherson region," he told Ukraine's Pryamyi TV channel.
"This is the announcement of what we have been waiting for since the spring — it is the beginning of the deoccupation of Kherson region."
Mr Khlan said Ukrainian troops now had "the advantage" on the southern front.
Several strikes in recent weeks have hit bridges in the region, in a bid to hamper logistics for the Russian military.
Ukrainian media had earlier quoted southern army command spokeswoman Natalia Gumeniuk as saying Kyiv's forces were attacking the front from "many directions".
On Facebook, the 'Kakhova' Ukrainian military group said it had seen "the retreat" of pro-Russian separatist fighters from their positions in the region.
None of these claims could be immediately verified by independent sources.
In late July, Mr Khlan said the region would be recaptured by Kyiv's forces by September.
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