The site — 200rf.com — contains pictures of the documents and corpses of Russian soldiers Ukraine said had been killed since President Vladimir Putin launched the attack.
It also has videos of soldiers Ukraine says it has captured.
“I am talking to you in Russian because this site was created for you,” Viktor Andrusiv, an adviser to the interior minister, said in a video posted on the site.
“I know that many Russians are worried about how and where their children, sons, husbands are and what is happening to them — so we decided to put this online so that each of you could search for your loved one who Putin sent to fight in Ukraine.”
Andrusiv said that over the past three days Ukrainian forces had captured almost 200 Russian soldiers and more than 3,000 Russian troops had died.
“We have documents, photos and videos of all of these people,” Andrusiv said.
The name of the site references the well-known term Gruz-200 (Cargo-200) that was used by Soviet military for corpses being flown back from the war in Afghanistan in the 1980s.
Russia’s defence ministry has so far given no details of any military losses in Ukraine since launching a multi-pronged attack Putin called a “special operation” to protect two separatist regions.
The head of the North Caucasus region of Dagestan, Sergei Melikov, on Saturday became the first official to report the death of a Russian soldier in Ukraine.
He posted a tribute on his official Instagram page, paying homage to an officer he said had been killed during the “special operation to defend Donbas”.
The Kremlin has launched a major propaganda campaign to control coverage of the war in Ukraine and has ordered media to use only Russia’s official versions of events.
Moscow has long been accused of covering up losses suffered by its forces as they backed pro-Russian separatists in Ukraine and fought in Syria.
Lev Shlosberg, a prominent liberal politician, has suggested Russia’s military was using mobile crematoriums to destroy evidence of those killed in Ukraine.
“There is no war. No dead. No tombs. People will just be no more. Forever,” he wrote on his blog.