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The Insider said Oksana Baulina, who previously worked for Russian opposition leader Alexey Navalny's anti-corruption group, "died under fire in Kyiv" while "filming the destruction" caused by Russian shelling.
"Insider journalist Oksana Baulina died during a bombardment in Kyiv while carrying out an editorial assignment ... another civilian died with her," the outlet, whose editorial offices are based in Latvia, said.
Ms Baulina was killed along with a civilian when Russian troops shelled a residential neighbourhood in Kyiv. Two civilians were injured, The Insider said.
The website said she left Russia after authorities designated Mr Navalny's Anti-Corruption Foundation an extremist organisation last year.
She continued reporting on corruption in Russia for The Insider and filed several reports from Kyiv and Lviv in western Ukraine for the outlet after Russian forces invaded the country a month ago, the news outlet said.
"The Insider expresses its deepest condolences to Oksana's family and friends," it said.
Ms Baulina's colleagues mourned her loss on social media.
Vladimir Milov, who worked with her at Mr Navalny's anti-corruption group, vowed to avenge her.
"I will never forget her and to all those who are responsible for her death I promise that they won't get away with [only] a trial and a verdict," Mr Milov said on Twitter.
"What an unbelievable horror," wrote Lyubov Sobol, another prominent member of Mr Navalny's team.
Sergiy Tomilenko, head of the Ukranian journalists' union, confirmed Ms Baulina's death in a statement on Facebook, saying she was reporting on the aftermath of a shelling when she was hit by fresh fire.
Earlier on Wednesday, the union said a cameraman for a local television station in the besieged southern city of Mariupol had also died.
At least five journalists have been killed since Russian forces invaded Ukraine on February 24. Other journalists killed include a US videographer, a French-Irish cameraman and a Ukrainian producer.