Iran acknowledged for the first time on Saturday that it had supplied Moscow with drones but said they were sent before the war in Ukraine, where Russia has used them to target power stations and civilian infrastructure.
Iranian Foreign Minister Hossein Amirabdollahian said a "small number" of drones had been shipped a few months before Russia's Feb 24 invasion.
Ukrainian President Volodymyr Zelenskiy accused Tehran of lying, saying Kyiv's forces were downing at least 10 of the unmanned aerial vehicles every day.
In Iran's most detailed response to date, Amirabdollahian denied Tehran was continuing to supply drones to Moscow.
"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 official IRNA news agency quoted him as saying.
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"The drone part is true and we provided Russia a small number of drones months before the Ukraine war," he said.
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.
In a video address, Zelenskiy 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.
Zelenskiy, who has repeatedly pressed allies to provide more defences against aircraft and missiles, said he expected "good news" in coming weeks but gave no details. Kyiv, he added, would next week launch a fund-raising campaign to buy marine drones.
Last month, two senior Iranian officials and two Iranian diplomats told Reuters Iran had promised to provide Russia with surface-to-surface missiles, in addition to more drones.
IRNA quoted Amirabdollahian as saying Tehran and Kyiv had agreed to discuss allegations about the use of Iranian drones but the Ukrainians did not show up for talks.
In a response on Facebook, Ukrainian Foreign Ministry spokesperson Oleg Nikolenko said Amirabdollahian was spreading "insinuations about an alleged refusal by the Ukrainian side".
Amirabdollahian repeated Tehran would "not remain indifferent" if it were proven Russia had used Iranian drones against Ukraine.
The European Union last month agreed new sanctions on Iran over drone deliveries, and Britain imposed sanctions on three Iranian military figures and a defence manufacturer.