Ukraine Friday downgraded diplomatic ties with Iran for supplying Russia with drones, a move President Volodymyr Zelenskiy called "a collaboration with evil".

Zelenskiy said a total of eight Iranian-made unmanned aerial (UAVs) vehicles had been shot down so far in the conflict.

US National Security Adviser Jake Sullivan warned in mid-July that Russia was preparing to acquire military drones from Iran. Later reports indicated a Russian delegation visiting Iran and assessing the potential of Iranian drones in its war in Ukraine.

In mid-August, Iran’s Revolutionary Guard, that controls most of the drone force, confirmed “joint exercises” with Russia in deployment of UAVs.

Iran indirectly denied American and Ukrainian accusations, claiming it was neutral in the war, but its ruler Ali Khamenei in a meeting with visiting Russian President Vladimir Putin praised his invasion of Ukraine as a justified act against the West.

"Today the Russian army used Iranian drones for its strikes. ... The world will know about every instance of collaboration with evil, and it will have corresponding consequences," Zelenskiy said in a late-night video address.

Military authorities in southern Ukraine said in a statement they had shot down four Shahed-136 "kamikaze" unmanned aerial vehicles over the sea near the port of Odesa on Friday.

The air force said separately that for the first time it had brought down a Mohajer-6, a larger Iranian drone, the Ukrainska Pravda newspaper reported.

The Ukrainian foreign ministry earlier said the supply of drones had dealt a serious blow to bilateral relations.

"In response to such an unfriendly act, the Ukrainian side has decided to deprive the ambassador of Iran of his accreditation and also to significantly reduce the number of diplomatic staff of the Iranian embassy in Kyiv," it said in a statement on its website.

The message was handed to the acting ambassador since the permanent envoy, Manouchehr Moradi, is not currently in Ukraine.

The Ukrainian military said earlier this month that an Iranian drone destroyed a US M777 howitzer provided to Ukraine. A report by the Wall Street Journal said that the Iranian drones were doing some damage to Ukrainian forces and Western military analysts have said that they can impact Kyiv’s war plans.

The issue is a challenge to the Biden administration that has been negotiating with Tehran to revive the Obama-era nuclear accord known as the JCPOA that former President Donald Trump abandoned in 2018.

Although negotiations are now deadlocked, but a new deal would lift key economic sanctions on Iran and provide tens of billions of dollars in financial relief that can enable Tehran to expand its military hardware and open a back door for Russia to circumvent Western sanctions.

Military experts say the drones would be useful to Russia for both reconnaissance and as loitering munitions that can bide their time in locating and engaging suitable targets.

In August, a senior U.S. administration official said Russia has faced "numerous failures" with Iranian-made drones acquired from Tehran.