Iran is believed to have sent more than 200 ballistic missiles to Russia - a move security chiefs say would be a "dramatic escalation" of its defence partnership with Moscow.
A Russian ship delivered the short-range Fatah-360 missiles from Tehran to a port in the Caspian Sea, a Ukrainian source told Sky News on Saturday.
Ukraine and its allies in the West have long feared that Iran has been supplying Russia with ballistic missiles.
Recap today's developments in Ukraine with our blog
So far it has supplied Vladimir Putin's forces with large quantities of attack drones - as well as artillery shells and ammunition.
But speaking at an event in London, CIA director Bill Burns said: "Should Iran ship ballistic missiles... it would be a dramatic escalation of the nature of the defence partnership."
Mr Burns also stressed how the Russia-Ukraine conflict demonstrates the impact of technology on the battlefield.
He and his UK counterpart, MI6 chief Sir Richard Moore accused Russia of a "reckless campaign of sabotage" in the unprecedented joint event on Saturday.
Meanwhile, military analyst Sean Bell said of the reported missile delivery: "It will mean Russia's limited supply of its own Iskander missiles can now be focused on long-range targets - that's very worrying."
He added: "We [also] understand that Russian soldiers have been in Iran doing training for the last few weeks."
Drone attacks on 11 Ukrainian regions
Meanwhile, in Ukraine overnight, dozens of drones were shot down across the country.
According to the Ukrainian Air Force, 67 drones were used across 11 regions, including Kyiv.
Fifty-eight of them were shot down with electronic weapons systems, they said.
The remnants of a kamikaze drone were seen outside the Ukrainian parliament building in central Kyiv on Saturday.
But the Ukrainian parliamentary press service said there had been no casualties and no damage to the building.
Elsewhere, three men were killed and three people injured in an artillery attack on the eastern city of Kostiantynivka.
Donetsk region governor Vadym Filashkin said the attack also damaged a high-rise building and local power lines.
It came after a week of long-range Russian attacks, including the missile strike on a military academy and hospital in Poltava, which killed 55 people and injured 328 others.
The funerals for some of the victims took place in the city on Saturday.
Meanwhile, Kyiv continued to launch its own strikes against Russia.
In the Russian border region of Voronezh, governor Aleksandr Gusev said that a drone strike started a fire and the detonation of "explosive objects" on Saturday.