"Our sense is you could include literally millions of rounds, rockets and artillery shells," National Security Council spokesman John Kirby told reporters.

Kirby, citing declassified US intelligence, stressed that the purchases were not yet completed.

"We don't have any indication that the purchase has actually occurred yet, so it's difficult to say what it's actually going to end up looking like," he said, adding that there are "certainly no indications that that materiel, those weapons, are being used inside Ukraine."

According to Kirby, massive purchases of artillery ammunition from the deeply isolated and repressive North Korean government, as well as a deal to purchase military drones from Iran, show the dire straits Russia faces after months of Western economic and technological sanctions aimed at crippling its war machine.

"It is just another indication of how desperate" President Vladimir Putin has become and "an indication of how much his defense-industrial establishment is suffering as a result," Kirby said. "The fact that they're having to buy artillery rounds from North Korea and drones from Iran tells you how effective it's been."

Kirby said there was no indication that China was cooperating with North Korea, saying "we continue to see no indication that China is violating sanctions with respect to Russia or in fact taking overt actions to assist Russia militarily."

Earlier, a US official said the ordnance was "for use on the battlefield in Ukraine."

"This purchase indicates that the Russian military continues to suffer from severe supply shortages in Ukraine, due in part to export controls and sanctions," the statement said.

Russia invaded Ukraine on February 24 with the apparent expectation of seizing control of the country within weeks.

But Ukraine has halted the advance with the help of weaponry and munitions from the United States and other NATO and European allies.

Both sides in the war have used huge amounts of artillery ammunition and lost large volumes of armor in the grinding battle.

The acquisition of longer-range missiles from the United States and allies has permitted Ukraine to target dozens of Russian ammunition depots behind the front lines.

And Western sanctions have made it harder for Moscow to acquire components to manufacture replacements, including computer chips.