South Korea believes North Korea has sent troops to Russia, marking a grave security threat to the international community that Seoul will respond to with all available means, the presidential office said in a statement on Friday.

Separately, South Korea's spy agency said the North was participating in the war in Ukraine and had decided to send 12,000 troops, including a special forces unit, Yonhap news agency reported.

South Korean President Yoon Suk Yeol held an unscheduled security meeting with key intelligence, military and national security officials to discuss North Korean troops' involvement in Russia's war against Ukraine, it said.

"The participants … shared the view that the current situation where Russia and North Korea's closer ties have gone beyond the movement of military supplies to actual dispatch of troops is a grave security threat not only to our country but to the international community," it said.

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South Korean officials have previously said it was likely true that some North Korean personnel were in Russia and involved in its war with Ukraine, but have not given a clear answer on the nature or the scale of any such deployment.

Yoon's office said South Korea together with its allies have been closely tracking North Korea's troop dispatch to Russia from the initial stages. It did not, however, provide any intelligence to back up the assertion of troop deployment.

It also did not specify if it had information on whether North Korean troops were involved in combat.

South Korea's National Intelligence Service could not be immediately reached for confirmation of the report on the number of North Korean troops.

Weapons sent to Russia, West believes

Yonhap said that the agency recently learned of the North Korean decision to deploy four brigades including a special forces unit and that the movement of troops has already started.

Ukraine's President Volodymyr Zelenskyy accused North Korea on Thursday of deploying officers alongside Russia and preparing to send thousands of troops to help Moscow's war effort, although NATO's chief Mark Rutte said there was no evidence of Pyongyang's presence at this stage.

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NATO showed no sign Wednesday it would invite Ukraine to join anytime soon as members of the security alliance sought more information from Ukrainian President Volodymyr Zelenskyy about his “victory plan” for ending the war with Russia.

Zelenskyy said about 10,000 North Korean soldiers are preparing for deployment to fight Ukraine, but Kyiv's Western allies have yet to confirm its assertion that Pyongyang is sending troops, though they say they are studying it.

Since their leaders' summit in the Russian far east last year, North Korea and Russia have dramatically upgraded their military ties and they met again in June to sign a comprehensive strategic partnership that includes a mutual defence pact.

North Korea has 1.28 million active duty troops, according to South Korea's latest data, and has stepped up its development of a series of ballistic missiles and a nuclear arsenal, fuelling regional tension and drawing international sanctions.

Deploying troops to Russia, if confirmed, would be its first major involvement in a war since the 1950-53 Korean War.

North Korea reportedly sent a much smaller contingent to the Vietnam War and to the civil conflict in Syria.

South Korean and U.S. officials have said North Korea has been supplying ballistic missiles and other munitions to Russia.

Since September last year, the North has shipped at least 16,500 containers of weapons to Russia and Russia has fired missiles from those shipments in Ukraine, Washington has said.

Russia and North Korea both deny they have engaged in arms transfers.

The Kremlin has also dismissed South Korean assertions that North Korea may have sent some military personnel to help Russia against Ukraine.