It is now "highly likely" that North Korean soldiers are "supporting Russia's war of aggression" against Ukraine, according to the UK government.
Speaking in the House of Commons, John Healey went further than previous ministers - including the prime minister - who only said the move was reported.
"In a concerning new development, it is now highly likely that the transfer of hundreds of combat troops from North Korea to Russia has begun," he said.
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Mr Healey added: "North Korean soldiers supporting Russia's war of aggression on European soil - it is as shocking as it is desperate."
Speaking just minutes previously, Foreign Office minister Stephen Doughty condemned what he called the "reported" actions by the reclusive nation - the full name of which is the Democratic People's Republic of Korea (DPRK).
Asked last week about the movement of DPRK troops, Sir Keir Starmer said that if the reports were true, it "shows a level of desperation" in Russia.
Foreign Secretary David Lammy has been in South Korea in the past few days.
In a joint statement released overnight, Mr Lammy and his South Korean counterpart Cho Tae-yul condemned the "reported" deployment.
Mr Lammy and Mr Cho also condemned in the "strongest terms" the "unlawful arms transfers" between Russia and the DPRK.
"Such cooperation between Russia and the DPRK is not only in violation of multiple UNSC resolutions, but also prolongs the suffering of the Ukrainian people and threatens global security, including those in the ROK and the UK, and demonstrates the desperation of the DPRK and Russia," the pair added.
"We are closely monitoring what Russia provides to the DPRK in return for its provision of arms and military personnel, including Russia's possible provision of materials and technology to the DPRK in support of Pyongyang's military objectives.
"We are also deeply concerned about the possibility for any transfer of nuclear or ballistic missile-related technology to the DPRK, which would jeopardize the international non-proliferation efforts and threaten peace and stability on the Korean Peninsula and across the globe."
Read more:
DPRK 'sends 1,500 troops to Russia'
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MPs in the House of Commons pushed Mr Healey to give them more details about the DPRK's presence in Russia.
The defence secretary said he was "not in a position to give the house any more details".
He added the UK is "watching this extremely closely, monitoring extremely closely" - noting there is a "growing cooperation" between both North Korea and Russia, and Iran and Russia, in the past 12 months.
(c) Sky News 2024: North Korean soldiers 'highly likely' to be supporting Russia in war against Ukraine, def
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