Iran admitted for the first time on Saturday that it had sent drones to Russia but insisted they were supplied to its ally before Moscow's invasion of Ukraine.

Kyiv and its Western allies have accused Russia of using Iranian-made drones in recent weeks to carry out attacks in Ukraine.

Tehran has repeatedly denied the claims but on Saturday Foreign Minister Hossein Amir-Abdollahian was quoted as saying that drones had been sent to Russia before the invasion began in late February.

"We supplied Russia with a limited number of drones months before the war in Ukraine," the official news agency IRNA quoted Amir-Abdollahian as saying.

For weeks, Russian forces have rained missiles and explosive drones onto Ukraine's infrastructure, as a major Ukrainian ground offensive -- propelled by Western arms deliveries -- has pushed Russian troops back in swathes of the country.

Russian strikes over the past month have destroyed around a third of Ukraine's power stations and the government has urged Ukrainians to conserve electricity as much as possible.

Ukraine's state energy company on Saturday announced additional power rationing in Kyiv and several other regions of the country.

"In a telephone conversation with the Ukrainian foreign minister last week, we agreed that if there was evidence (of Moscow's use of Iranian drones), he would provide it to us," Amir-Abdollahian said.

"If the Ukrainian side keeps its promise, we can discuss this issue in the coming days and we will take into account their evidence," he added.

And he again denied Iran had supplied missiles to Russia, calling the accusations "completely false".

In response, Ukraine's foreign ministry spokesman warned Iran Saturday in a post on Facebook that "the consequences of complicity" with Moscow would be "greater than the benefit from Russia's support."