U.S. Defense Secretary Lloyd Austin said there is evidence that North Korea has sent troops to Russia on Monday, and South Korea's spy chief told lawmakers that 3,000 North Korean troops are in the country receiving training on drones and other equipment before being deployed to battlefields in Ukraine.

Austin told reporters Wednesday "What exactly they are doing? Left to be seen. These are things that we need to sort out," according to a video posted by the Washington Post.

If the troops join the war in Ukraine on Russia's side, it will be "a very, very serious issue," Austin said, adding it would have an impact in Europe and in the Indo-Pacific region.

South Korean intelligence first publicized reports that the Russian navy had taken 1,500 North Korean special warfare troops to Russia last week, while Ukrainian President Volodymyr Zelenskyy had earlier said his government had intelligence that 10,000 North Korea soldiers were being prepared to join the invading Russian forces.

The U.S. and NATO had not previously formally confirmed North Korea's reported troop dispatch, but have warned of the danger of such a development if true. Russia and North Korea have so far denied the troop movements.

South Korean National Intelligence Service Director Cho Tae-yong told lawmakers that said another 1,500 North Korean troops have entered Russia in a closed-door meeting, according to lawmaker Park Sunwon, who attended the briefing.

Cho told lawmakers that his agency assessed that North Korea aims to deploy a total of 10,000 troops to Russia by December, Park told reporters.

Park cited Cho as saying the 3,000 North Korean soldiers sent to Russia have been split among multiple military bases and are in training. Cho told lawmakers that NIS believes they have yet to be deployed in battle, according to Park.

Speaking jointly with Park about the NIS briefing, lawmaker Lee Seong Kweun said that the NIS found that the Russian military is now teaching those North Korean soldiers how to use military equipment such as drones.

Lee cited the NIS chief as saying Russian instructors have high opinions of the morale and physical strength of the North Korean soldiers but think they will eventually suffer a heavy causalities because they lack an understanding of modern warfare. Lee, citing Cho, said Russia is recruiting a large number of interpreters.

Lee said NIS has detected signs that North Korea is relocating family members of soldiers chosen to be sent to Russia to special sites to isolate them.

The NIS chief told lawmakers that North Korea hasn't disclosed its troop dispatch to its own people. But there are rumors that the news is spreading to local residents, including those whose loved ones have been assigned Russian tours, Lee said, citing the NIS.

Ukraine's Military Intelligence Directorate head, Kyrylo Budanov, told the online military news outlet The War Zone that North Korean troops will arrive to Russia's Kursk region today to help Russian troops fighting off a Ukrainian incursion.

North Korea and Russia, embroiled in separate confrontations with the West, have been sharply boosting their cooperation in the past two years. In June, they signed a major defense deal requiring both countries to use all available means to provide immediate military assistance if either is attacked.

The NIS said last week that North Korea had sent more than 13,000 containers of artillery, missiles and other conventional arms to Russia since August 2023 to replenish its dwindling weapons stockpiles.

Reports that the North is sending troops to Russia stoked security jitters in South Korea. South Korean officials worry that Russia may reward North Korea by giving it sophisticated weapons technologies that could boost the North's nuclear and missile programs that target South Korea.

South Korea said Tuesday it would consider supplying weapons to Ukraine in response to the North's reported troop dispatch. South Korea has shipped humanitarian and financial support to Ukraine, but it has so far avoided directly supplying arms to Ukraine in line with its policy of not supplying weapons to countries actively engaged in conflicts.

North Korea has 1.2 million troops, one of the largest standing armies in the world, but it hasn't fought in large-scale conflicts since the 1950-53 Korean War. Many experts question how much North Korean troops would help Russia, citing a shortage of battle experience. They say North Korea wants to get Russian economic support and its help to modernize the North's outdated conventional weapons systems as well as its high-tech weapons technology transfers.