South Korea said on Friday that it 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.

Separately, the country's spy agency said North Korea 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.

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 backup 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.

Ukraine's President Volodymyr Zelenskiy 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.

Zelenskiy 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.

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.