The incident marked the second apparent breach within a week of the tightly controlled war narrative that the Kremlin promotes through loyal Russian media.
An online article on the site of mass-market paper Komsomolskaya Pravda, still accessible via a web archive tool, quotes the Russian defence ministry as saying 9861 Russian servicemen have been killed and 16,153 wounded in what it calls its special military operation in Ukraine.
Those figures had been removed from a version of the same article visible on the website on Tuesday.
Instead, an advisory said: "On March 21, access to the administrator interface was hacked on the Komsomolskaya Pravda website and a fake insert was made in this publication about the situation around the special operation in Ukraine. The inaccurate information was immediately removed."
Russia has not officially updated its casualty figures since stating on March 2 that 498 servicemen had been killed and 1597 wounded.
Since then its offensive has run into further heavy resistance from Ukraine's army and volunteer defence forces.
Kremlin spokesman Dmitry Peskov told reporters on a conference call on Tuesday that he could not comment on the incident with Komsomolskaya Pravda, saying it was a question for the newspaper.
He said he had no information on casualty figures.
Alexander Gamov, a Kremlin correspondent for the paper, said on the same call that its website had been hacked and fake information had appeared there for several minutes.
Earlier, Ukrainian presidential adviser Mykhailo Podolyak drew attention to the two online versions of the newspaper article and the alleged figure of 9861 Russian deaths.
"This is only the beginning of the realisation of their national catastrophe. Because in the real world there are almost twice as many as Russians killed," Podolyak wrote on Telegram.
It was not possible to independently verify any of the purported casualty claims.
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