Ukraine said Friday it was so far unable to confirm that Russia had used North Korean missiles to attack it after Washington accused Moscow of using weapons provided by Pyongyang.
The White House laid out the claims late Thursday and also said Moscow was seeking missiles from its ally Iran, as it burns through stockpiles of key munitions nearly two years into its invasion of Ukraine.
US National Security Council spokesman John Kirby described the deliveries as a "significant and concerning escalation" but Kyiv hours after the announcement said Ukraine had yet to independently find evidence to support the claims.
"So far, we have no information that such missiles have been used. The United States made a statement to that effect," Ukraine's air force spokesman Yuriy Ignat told state media.
"So experts will study the wreckage, and then we can say whether this is a fact or not. I can't confirm it yet," he added.
In their grinding war of attrition, which the Kremlin had hoped would conclude in just days, Kyiv and Moscow have struggled to refill stockpiles of artillery shells, drones and long-range missiles.
The Pyongyang-supplied missiles with ranges of around 900 kilometres (550 miles) were fired by Russia in two attacks on Ukraine within the past week, Kirby said.
He said that Russia had launched at least one of the North Korean-supplied missiles on December 30, which landed in an open field in the Zaporizhzhia region.
Moscow's forces then fired "multiple" ballistic missiles into Ukraine as part of a mass aerial attack on January 2, he added.
Russia rained down missiles on Ukraine's two largest cities, Kyiv and Kharkiv, on that date, in fatal strikes that led Kyiv to hit back at the Russian border city of Belgorod, forcing schools to close.
Kirby said the development underscored the need for the US Congress to pass a package of vital military aid for Ukraine, without which the pipeline of support will soon run dry, Kirby added.
Air defences for Ukraine, which has come under a barrage of aerial attacks by Russia in recent days, were "absolutely" a priority, he said.
The United States said in October that North Korea had delivered more than 1,000 containers of military equipment and munitions to Russia, but it was the first time it had reported the use of such weapons.
Ukraine has substantially boosted its air defence systems compared to last winter, when systematic Russian strikes on energy infrastructure left millions in the cold and dark for extended periods.
Ignat's response to the White House's claims on Friday came hours after Russia's defence ministry said its forces had repelled a Ukrainian attack over Crimea, shooting down 36 drones over the peninsula annexed in 2014.
Kyiv has struck the Russian-controlled territory throughout the war, launching missiles and drones at Sevastopol, Crimea's largest city, in a bid to suppress Russia's Black Sea naval fleet.
