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.

"The Russian Ministry of Defense is in the process of purchasing millions of rockets and artillery shells from North Korea for use on the battlefield in Ukraine," a US official said in a statement.

"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 official said, on grounds of anonymity.

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

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

Under that pressure, the US official said, "We expect Russia could try to purchase additional North Korean military equipment going forward."