North Korea has shipped 1,500 special forces troops to Russia's Far East for training at local military bases, and they will likely be deployed for combat in the war in Ukraine, South Korea's spy agency said on Friday.

South Korea's National Intelligence Service said it had been working with the Ukrainian intelligence service and had used facial recognition artificial intelligence technology to identify North Korean officers in eastern Ukraine's Donetsk region supporting Russian forces firing North Korean missiles.

North Korea has shipped artillery rounds, ballistic missiles, and anti-tank rockets to Russia in more than 13,000 containers since August last year, the agency said, based on the remnants of weapons recovered from the battlefront in Ukraine.

In all, more than 8 million artillery and rocket rounds have been shipped to Russia, it said.

"The direct military cooperation between Russia and North Korea that has been reported by foreign media has now been officially confirmed," the spy agency said in a statement.

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

"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," Yoon's office said.

It said South Korea, together with its allies, has been closely tracking North Korea's troop dispatch to Russia from the initial stages. South Korea will respond to the North's activities with all available means, it added, without elaborating on what actions it might take.

South Korea, which has emerged as a major global arms exporter, selling fighter jets, mechanized howitzers, and missiles, has come under pressure from some Western allies including Washington to help arm Ukraine with lethal weapons but has stopped short of openly doing so.

The House of Representatives intelligence committee chairman cited the South Korean report in a letter to President Joe Biden calling for an immediate classified briefing to the panel on the issue.

"These [North Korean] troops movements, if true, are alarming and are an extreme escalation of the conflict in Ukraine," wrote Rep. Mike Turner, R-Ohio. "They require an immediate response from the United States and our NATO allies to avoid a widening conflict."

The White House National Security Council could not confirm reports that North Korean troops were fighting for Russia, a spokesperson said; but if true, "this would mark a dangerous development in Russia's war against Ukraine."

Ramon Pacheco Pardo of King's College in London said despite the gravity of the development, it may not be heavy enough to shift Seoul's position.

"When it comes to South Korea, I think that its red line is Russia providing support to North Korea that allows Pyongyang to substantially improve its nuclear and missile program, not North Korea's support for Russia."

Russian Uniforms, Fake IDs

Vessels belonging to Russia's Pacific Fleet were detected moving about 1,500 North Korean special forces troops to Vladivostok from Oct. 8 to 13 and are expected to resume the shipment of troops soon, South Korea's intelligence service said.

The troops have been supplied with Russian military uniforms and weapons as well as fake identification documents for when they are deployed for combat, it said.

The agency said it used facial recognition AI to identify with a high degree of accuracy technical military officers from the North Korean military in Russian-occupied regions of Ukraine where they are supporting Russia's missile offensive and helping with technical glitches.

Ukrainian President Volodymyr Zelenskyy accused North Korea on Thursday of deploying officers alongside Russia and preparing to send 10,000 soldiers to help Moscow's war effort, although NATO chief Mark Rutte said there was no evidence of Pyongyang's presence at this stage.

Since their leaders met in Russia's Far East last year, North Korea and Russia have dramatically upgraded their military ties. They met again in June to sign a comprehensive strategic partnership that includes a mutual defence pact.

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.

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.