The European Union said Monday its allies had shared intelligence Iran had supplied Russia with ballistic missiles, a claim that Tehran rejected but that the Kremlin did not explicitly deny.
US media outlets reported last week that Washington believed Iran had transferred the weapons to Russia for use on the battlefield in Ukraine, citing anonymous sources.
"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".
Iran's foreign ministry said it "strongly" rejected accusations it had a role in exporting arms to Russia.
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.
"We strongly reject the claims on Iran's role in exporting arms to one side of the war," Iranian foreign ministry spokesman Nasser Kanani said during a weekly press conference.
The Kremlin did not issue a denial on Monday when asked specifically about the Wall Street Journal 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 United States has said any deliveries would invoke a "severe" response and damage Tehran's efforts to improve relations with the West following the country's election of reformist Masoud Pezeshkian as president.
Faced with punishing Western sanctions, Moscow has turned to Iran and North Korea for weapons supplies to keep its war machine going in Ukraine.
Ukraine says it has been attacked with Iranian-designed Shahed drones on an almost daily basis from Russia, and has found fragments of North Korean missiles on its territory.
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.
