The Kremlin on Tuesday mocked opposition politician Alexei Navalny and tried to call his sanity into question, a day after he said he had tricked a Russian secret agent into disclosing lurid details of a botched plot to kill him.

Navalny, one of President Vladimir Putin's most outspoken critics, was airlifted to Germany for medical treatment in August after collapsing on a plane in Russia.

Germany has said he was poisoned with a Soviet-style Novichok nerve agent in an attempt to murder him, an assertion many Western nations accept.

The Kremlin has repeatedly rejected any suggestion that Russia tried to kill Navalny.

Navalny on Monday released a recording of him speaking by phone at length to a man he described as a state security agent who told him, among other things, that the poison had been placed in his underpants.

Navalny said he had posed as an aide to a senior Russian security official to gain the man's confidence. Russia's FSB security service dismissed the recording as a fake.

Dmitry Peskov, Putin's spokesman, told reporters on a conference call on Tuesday he was speaking in a personal capacity, but thought Navalny was suffering from psychological issues.

“We can say that the patient [Navalny] has a pronounced persecution mania. You can also clearly identify certain signs of megalomania,” said Peskov.

Kira Yarmysh, Navalny's spokeswoman, said on Twitter that it looked like Peskov had been forced to improvise and mocked what she portrayed as the Kremlin's attempts to obscure the truth.

She pointed out that Putin himself had recently confirmed that state security agents had kept an eye on Navalny, something she suggested sat oddly with the Kremlin now accusing him of having persecution mania.