He declined to provide a specific estimate for the amount of ammunition allegedly being sent to bolster the Russian effort.
Mr Kirby said North Korea “is covertly supplying” the ammunition to Russia, but that “we’re still monitoring this to determine whether the shipments are actually received”.
He insisted that the North Korean shipments are “not going to change the course of the war,” citing Western efforts to resupply the Ukrainian military.
The White House would not specify the mode of transportation or whether the US or other nations would attempt to interdict the shipments to Russia.
Iran is also accused of supplying arms to Russia during its war with Ukraine, which began on February 24.
Ukraine and its Western allies have accused Iran of sending “kamikaze” drones which have then been used to devastating effect by Russian forces in strikes targeting crucial energy infrastructure in Ukraine. Iran denies the charge.
Ukraine’s Foreign Minister Dmytro Kuleba last week demanded Tehran stop sending weapons to Russia.
But Iran’s Foreign Minister Hossein Amirabdollahian repeated Iran’s denial of supplying the warring country weapons, according to Iranian state media.
“We have good ties with Russia and have had defence cooperation since earlier, but our policy towards the war in Ukraine is to respect the territorial integrity of countries, not to send weapons to conflicting parties, stop the war and end the displacement of people,” Amirabdollahian was quoted as saying.