Dmitry Peskov, spokesman for Russian President Vladimir Putin, spoke for the first time about the resignation of Russian diplomat Boris Bondaryev.
The adviser to Russia's Permanent Mission to the United Nations in Geneva described the war as a "bloody, senseless and absolutely unnecessary disgrace" in which he no longer wanted to be a part.
The letter – in which Bondaryev had explained his motives for the withdrawal – was not known in the Kremlin, Peskov said, referring to the Russian Foreign Ministry, which has not yet commented on the matter.
"One can probably only say here that Mr. Bondaryev is no longer with us, but rather against us," said Peskov.
Frederick Smith
Political correspondent for Russia and the CIS in Moscow.
Follow I follow
When asked what "against us" meant, Putin's spokesman said: "He condemns the actions of the Russian leadership, and the actions of the Russian leadership are supported by practically the entire population of our country." Peskov's comments are the first official Russian reaction to Bondaryev's resignation.
He had told the BBC that he saw "no alternative" to the move.
His colleagues initially greeted the invasion of Ukraine with "happiness, joy, euphoria" but are now "less happy with it because we are facing some problems, especially with the economy".
But he doesn't see that many of his colleagues would regret it and change their minds, Bondaryev said.
According to the BBC, he is aware that Moscow now sees him as a "traitor".
Bondaryev said that even though he "didn't do anything illegal," he now had to worry "about my safety."