WASHINGTON -- North Korea has delivered more than 1,000 containers of military equipment and munitions to Russia in recent weeks for use in Ukraine, a White House official said Friday, lambasting burgeoning cooperation between Pyongyang and Moscow as a threat to regional stability.

In a press briefing, John Kirby, National Security Council coordinator for strategic communications, disclosed the information, amid persistent speculation that the Sept. 13 summit between North Korean leader Kim Jong-un and Russian President Vladimir Putin might have led to an arms deal.

Kirby also voiced concerns about possible Russian assistance to the North in return for arms support, disclosing the US' observation that Russian ships offloaded containers in the North, which he said "may constitute the initial deliveries of material from Russia."

"Our information indicates that in recent weeks, North Korea has provided Russia with more than 1,000 containers of military equipment and munitions," he said.

The White House released imagery showing that between Sept. 7 and Oct. 1, a set of containers were shipped from Najin, North Korea, to Dunay, Russia, via MV Angara, a Russian-flagged cargo ship. The shipping containers were then moved via rail to an ammunition depot in southwestern Russia near Tikhoretsk, roughly 290 kilometers from the Ukrainian border.

One of the photos also showed a shipment of about 300 shipping containers at Najin for delivery to Russia on Sept. 7-8 -- an indication that a decision on such an arms transfer was made prior to the rare meeting between Kim and Putin at the Vostochny Cosmodrome, a Russian spaceport.

"We condemn the DPRK for providing Russia with this military equipment, which can be used to attack Ukrainian cities and kill Ukrainian civilians, and for the Russians and illegitimate war," Kirby said, referring to North Korea by its official name, the Democratic People's Republic of Korea.

The official also raised the possibility that Russia could offer something in exchange.

"We also are increasingly concerned about Russian assistance to the DPRK," he said. "In return for support, we assess that Pyongyang is seeking military assistance from Russia, including fighter aircraft, surface-to air missiles, armored vehicles, ballistic missile production equipment, or other materials and other advanced technologies."