"We are aware of the credible information provided by allies on the delivery of Iranian ballistic missiles to Russia," EU spokesman Peter Stano said.

"We are looking further into it with our member states and if confirmed, this delivery would represent a substantive material escalation in Iran's support for Russia's illegal war of aggression against Ukraine."

Stano added that "the EU leaders' unanimous position has always been clear. The European Union will respond swiftly and in coordination with international partners, including with new and significant restrictive measures against Iran".

The Wall Street Journal reported Friday that the United States had briefed European allies that Iran has delivered short-range short-range ballistic missiles to Russia.

"We strongly reject the claims of Iran's role in exporting arms to one side of the war," Iranian foreign ministry spokesman Nasser Kanani told a news conference.

"Iran's accusers are the ones who are among the biggest arms exporters to one side of the war," he added, reiterating that Tehran "is not part of the war" in Ukraine.

The Kremlin did not issue a denial on Monday when asked about the report that Iran had sent missiles.

"We have seen this report, it is not every time that this kind of information is true," Kremlin spokesman Dmitry Peskov said.

"Iran is our important partner, we are developing our trade and economic relations, we are developing our cooperation and dialogue in all possible areas, including the most sensitive areas."

The West has been warning Tehran against sending Russia missiles for months, and the EU has already repeatedly hit Iran with sanctions for supplying drones to Moscow for the war in Ukraine.

The reported delivery of missiles to Russia comes as the Kremlin has once again stepped up its bombing campaign against Ukraine's key infrastructure ahead of winter.

Faced with punishing Western sanctions, Moscow has turned to Iran and North Korea for weapons supplies to keep its war machine going in Ukraine.

In August, Russian national security council secretary Sergei Shoigu headed a delegation to Tehran for meetings with Iranian counterpart, Ali Akbar Ahmadian, and Iranian President Masoud Pezeshkian.

The visit had the aim of "strengthening interactions, the examination of regional and international issues, and bilateral political security relations," according to Iran's Tasnim news agency