Ukraine President Volodymyr Zelensky has accused Iran of "lying" and "terrorist cooperation" by supporting Russia's war in Ukraine.
It comes after Iran admitted for the first time it had supplied drones to Moscow, but only before the invasion.
The West believes Russia has used Iranian "kamikaze" drones to hit key infrastructure sites in Ukraine, but Iran and Moscow previously denied it.
Blackouts have hit much of Ukraine after the attacks on power stations.
Western officials have accused Iran of supplying drones to Russia and of providing personnel on the ground to train Russian drone pilots.
Previously, Tehran denied the accusations, but on Saturday Iran's foreign minister said a small number of the vehicles had been sent to Moscow.
"We provided a limited number of drones to Russia many months prior to the Ukraine war," Hossein Amir-Abdollahian told reporters in Tehran.
However, in an address on his Telegram channel, President Zelensky accused Iran of lying "even in this confession".