Russia has been forced to buy military hardware from North Korea as sanctions squeeze Moscow's ability to supply its military, the US says.
A US official revealed Moscow is in the process of purchasing millions of rockets and artillery shells from Pyongyang for use in Ukraine.
And they said that Russia could be forced to buy additional North Korean weaponry as the war dragged on.
Buying from North Korea is a sign of "severe supply shortages", they added.
The intelligence was first reported by the New York Times. Ukraine's defence ministry tweeted a mocking response to the report, saying that it showed "Soviet weapons" had "exhausted their potential".
The ministry said the while Ukraine was switching to Nato standards, Russia was heading in the direction of North Korean standards - be it in the field of "weapons, politics or standard of living".
Kim Jong-un's regime has blamed the US for the conflict and accused the West of pursuing a "hegemonic policy" that justified Russia's use of force.
Last month, North Korea recognised the independence of Russia's two proxy statelets in eastern Ukraine - the Donetsk and Luhansk People's Republics - and vowed to deepen its "comradely friendship" with Moscow.
Russia's Vladimir Putin said the two countries would expand their "comprehensive and constructive bilateral relations", according to Pyongyang state media.
Broad economic sanctions have done little to damage Russia's income from energy exports, according to Finnish think tank the Centre for Research on Energy and Clean Air.
It estimates Russia has made €158bn (£136bn) from surging fossil fuel prices during the six-month invasion, with EU imports accounting for more than half of that.
But the US thinks export controls and sanctions are affecting the Russian military.
Last week, officials in the Biden administration told US media that the first shipments of Iranian-made drones had also been delivered to Russia.
US intelligence officers believe that Russian operators have travelled to Iran to receive training on the Mohajer-6 and Shahed series weapons.
But they told reporters recently that many of the drones had been beset by mechanical and technical problems since delivery.
Iran has officially denied delivering weapons to either side of the conflict, but in July US National Security Advisor Jake Sullivan said Tehran was planning to supply Moscow with potentially hundreds of drones for its war in Ukraine, some with combat capabilities.
On Tuesday, UK defence officials said in a daily update that Russia was struggling to maintain its supply of battlefield drones in the face of significant "combat losses".
In other developments in Ukraine on Tuesday:
- New Russian missile strikes have been reported across the country, with a fuel depot set on fire in the Kryvyi Rih area in central Ukraine. In a photo posted by the governor of Dnipropetrovsk region thick black smoke could be seen billowing from the depot. It was attacked with two missiles on Monday evening, Valentyn Reznichenko said, adding that there was no immediate information about casualties
- In the north-eastern city of Kharkiv, a woman's body was found after a strike destroyed the upper part of a block of flats, local authorities said
- Russian-backed separatists controlling Donetsk said parts of the eastern city had been shelled by Ukrainian government forces on Tuesday, with one civilian wounded.