The report by the Pentagon's Defense Intelligence Agency used open-source imagery to confirm that debris found in Ukraine's Kharkiv region in January this year is from a short-range ballistic missile made in North Korea.

"Analysis confirms that Russia used ballistic missiles produced in North Korea in its war against Ukraine," the DIA said in a statement released with the report on Wednesday.

"North Korean missile debris was found throughout Ukraine," it added.

South Korea accuses Pyongyang of sending thousands of containers of munitions to Russia, which would violate rafts of United Nations sanctions on both countries.

The powerful sister of North Korean leader Kim Jong Un this month denied the allegations that Pyongyang is shipping weapons to Russia, calling the claim "absurd".

Pyongyang has "no intention to export our military technical capabilities to any country," Kim Yo Jong said.

But experts maintain that a recent testing spree -- which has seen the North repeatedly fire off rockets, cruise and ballistic missiles -- may be of weapons destined for use on battlefields in Ukraine.

The DIA report compares images in North Korean state media to other photographs showing missile debris in Ukraine's Kharkiv region in January.

South Korea's defence ministry told AFP it has no comment to give on the report.

Pyongyang and Russia have moved to boost ties in recent months.

Kim Jong Un visited Russia last year for a summit with Putin to cement the deepening ties, in a rare foreign trip for the reclusive North Korean leader.

The Kremlin told Russian media this month that a return visit by Putin to North Korea was "being prepared".

Putin last visited Pyongyang in 2000, just months after entering the Kremlin, for a meeting with Kim Jong Il, Kim Jong Un's father and predecessor.

North Korea is barred by UN sanctions from any tests using ballistic technology.

But Moscow used its UN Security Council veto in March to effectively end UN monitoring of violations, for which Pyongyang has specifically thanked Russia.