A declassified U.S. intelligence assessment says Russia is acquiring millions of artillery shells and rockets from North Korea for its six-month-old invasion of Ukraine, in an indication of the effect that Western sanctions are having on the Russian war effort.

The New York Times published the findings but said the specifics of such shipments including their type and timing were unclear.

The paper quoted an unnamed U.S. official as saying such purchases were expected to continue and perhaps expand beyond short-range rockets and ammunition.

The disclosure of the U.S. assessment follows reports last month originating from U.S. sources asserting that there were mechanical or technical problems among the first two types of military drone purchased recently from Russian ally Iran.

Russian President Vladimir Putin and his intelligence chiefs and war planners are thought to have believed that the main aims of their full-scale invasion after eight years of war mostly by proxy in eastern Ukraine could be achieved in a matter of days when they launched it in late February.

But Ukrainians have mounted a fierce defense and, aided by Western weapons shipments, have recently launched a major counteroffensive to retake territory in southern Ukraine.

Both sides' casualty figures are classified, but the consensus is that each side has lost tens of thousands of soldiers or, in Ukraine's case, troops along with civilian defense forces.

Kyiv and Moscow have both pledged to fight as long as it takes to secure victory.

Western governments and NATO members have supplied tens of billions of dollars in weapons and other military aid to Ukraine for a campaign that some regard as a potential Kremlin stepping stone to further wars of territorial expansion, particularly among former Soviet republics.