The European Union said Monday it had imposed sanctions on nine Russian officials involved in the jailing of Kremlin critic Vladimir Kara-Murza.

Kara-Murza, 41, was in April sentenced to 25 years in a high-security prison on treason and other charges for criticising Russia's war in Ukraine.

The sentence was the longest given to a Russian opposition figure in recent years.

The "outrageously harsh prison sentence clearly demonstrates the political misuse of the Russian judiciary in order to suppress civil society and independent voices opposing Russia's illegitimate war of aggression against Ukraine", said EU foreign policy chief Josep Borrell in a statement.

Among those sanctioned were the deputy justice minister, judges, and other members of the judiciary, as well as a senior prison official "responsible for the degrading treatment, which has led to a considerable deterioration of Kara-Murza's health", said the statement.

They have had their assets frozen and are banned from travelling to or through EU countries.

The United Kingdom and Lithuania have also sanctioned high-ranking Russian officials over the trial.