Iran on Saturday admitted for the first time that it supplied Russia with drones but claimed they were sent before the war erupted in February.
Iranian Foreign Minister Hossein Amirabdollahian said a “small number” of drones had been supplied to Russia a few months before Moscow’s forces invaded Ukraine, on February 24.
But during his televised address to Ukraine on Saturday night, President Volodymyr Zelensky accused Iran of “lying about the obvious”, saying Kyiv’s forces were downing at least 10 of the unmanned aerial vehicles every day.
His comments came after Iran's most detailed response on the issue to date, in which Mr Amirabdollahian said, according to the official IRNA news agency: "This fuss made by some Western countries that Iran has provided missiles and drones to Russia to help the war in Ukraine - the missile part is completely wrong.
"The drone part is true and we provided Russia a small number of drones months before the Ukraine war.”
Mr Amirabdollahian said Iran remained committed to stopping the conflict.
“If (Ukraine) has any documents in their possession that Russia used Iranian drones in Ukraine, they should provide them to us,” he said.
“If it is proven to us that Russia used Iranian drones in the war against Ukraine, we will not be indifferent to this issue.”
In his Saturday video address, Mr Zelensky dismissed talk of limited Iranian supplies to Russia, saying Ukraine had downed 11 drones on Friday alone.
"If Iran continues to lie about the obvious, it means the world will make even more efforts to investigate the terrorist cooperation between the Russian and Iranian regimes and what Russia pays Iran for such cooperation," he said.
Separately, US Special Envoy for Iran Robert Malley tweeted it was untrue that Iran had sent a few drones.
"They transferred dozens just this summer and have military personnel in occupied Ukraine helping Russia use them," he said.
Previously, Iranian officials had denied arming Russia in its war on Ukraine.
Earlier this week, Iran’s Ambassador to the UN Amir Saeid Iravani called the allegations “totally unfounded” and reiterated Iran’s position of neutrality in the war.
The Security Council had called on Secretary-General Antonio Guterres to investigate if Russia has used Iranian drones to attack civilians in Ukraine.
In recent weeks Ukraine has reported a surge in drone attacks on civilian infrastructure, notably targeting power stations and dams, using Iranian-made Shahed-136 drones. Russia denies its forces have used Iranian drones to attack Ukraine.
IRNA news reports that Mr Amirabdollahian said Tehran and Kyiv had agreed to discuss allegations about the use of Iranian drones in Ukraine two weeks ago but the Ukrainians did not show up at the agreed meeting.
“We agreed with the foreign minister of Ukraine to provide us with documents they have that Russia used Iranian drones in Ukraine,” Mr Amirabdollahian said, but the Ukrainian delegation pulled out of the planned meeting at the last minute.
The Iranian foreign minister repeated that Tehran would “not remain indifferent” if it were proven that Russia had used Iranian drones in the war against Ukraine.
The European Union last month agreed new sanctions on Iran over its drone deliveries to Russia, and Britain imposed sanctions on three Iranian military figures and a defence manufacturer for supplying Russia with drones to attack civilian and infrastructure targets in Ukraine.