"We do have indications that Russia has approached North Korea to request ammunition," Pentagon spokesman Brigadier General Pat Ryder said.

A separate statement from a US official said the purchase would involve rockets and artillery shells "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.

But 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.

Ryder said the purchase from North Korea underscores the "challenges" the Russians have in logistics and supply chains for its forces inside Ukraine.