The number far exceeds the figure US officials have previously provided.
"It is clear that Russian and North Korean military leaders are treating these troops as expendable and ordering them on hopeless assaults against Ukrainian defences," Kirby said on Friday, describing the North Korean troops' offensive as "massed, dismounted assaults."
US President Joe Biden condemned Russia's Christmas Day attacks on Ukraine, but fighting continues. (AP PHOTO)
North Korea's mission to the United Nations in New York did not immediately respond to a request for comment, and Russia's UN mission declined to comment.
Earlier this week US President condemned Russia's Christmas Day attacks on Ukraine's energy system and some of its cities and asked the Defence Department to continue its surge of weapons to Ukraine.
On December 17, a US military official said North Korea had suffered several hundred casualties while fighting against Ukrainian forces in Russia's Kursk region.
US President Joe Biden has asked the Defence Department to continue its surge of weapons to Ukraine. (AP PHOTO)
Asked about what ranks the North Korean casualties included, the military official, speaking on the condition of anonymity, said it was from lower-level troops to "very near to the top."
In his nightly video address, Ukrainian President Volodymyr Zelenskiy said Russian troops were sending the North Koreans into battle with minimal protection and that they were taking extreme measures to avoid being taken prisoner.
"Their losses are significant, very significant. We see that neither the Russian military nor their North Korean overseers have any interest in ensuring the survival of these North Koreans," he said.
"Everything is set up so that it is impossible for us to capture them. There are instances in which they are executed by their own forces. Russians send them into assaults with minimal protection."
Volodymyr Zelenskiy said Korean people "should not be losing their lives in battles in Europe". (EPA PHOTO)
Ukrainian and Western intelligence reports say there are about 12,000 North Korean troops in Kursk, a Russian region on the border where Ukrainian forces are holding chunks of territory after staging an incursion in August.
Earlier this week, Zelenskiy said more than 3000 North Koreans had been killed or wounded.
He said Ukrainian forces had managed to take a few North Korean soldiers prisoner "but they were severely wounded and it was not possible to save their lives".
Zelenskiy said the Korean people "should not be losing their lives in battles in Europe. This is something that Korea's neighbours, including China, can influence."
"If China is sincere in its statements that the war should not expand, it needs to exert appropriate pressure on Pyongyang."
Reuters was unable to independently verify reports of combat losses.
Separately, the United States is expected to announce that it will send $US1.25 billion ($A2.01 billion) in military assistance to Ukraine as the Biden administration pushes to get as much aid to Kyiv as possible before leaving office on January 20.
The large package of aid includes a significant amount of munitions, including for the National Advanced Surface-to-Air Missile Systems and the Hawk air defence system.
The US aid packages includes surface to air missile systems. (AP PHOTO)
It also will provide Stinger missiles and 155mm and 105mm artillery rounds, officials said.
The officials, who said they expect the announcement to be made on Monday, spoke on condition of anonymity to provide details not yet made public.
with PA
