Russia has supplied air defence missiles to North Korea in exchange for sending its troops to support Russia's war efforts against Ukraine, a top South Korean official said Friday.

The US, South Korea and Ukraine say North Korea sent more than 10,000 troops to Russia in October, some of whom have recently began engaging in combat. It has been unclear what Russia would give North Korea in return.

Shin Wonsik, national security adviser for President Yoon Suk Yeol, told a SBS TV program Friday that South Korea has found Russia provided missiles and other equipment to reinforce its air defence network for Pyongyang, the capital.

Many observers say North Korea likely feels the urgent need to boost its air defence capabilities for the capital after the North last month accused South Korea of dropping propaganda leaflets over Pyongyang. North Korea threatened to take military action if leaflets were again dropped. South Korea's military has refused to confirm whether or not it was behind the alleged drone flights.

Shin says Russia has also give economic assistance to North Korea and various military technologies, including those needed to help the North build a reliable space-based surveillance system.

During a summit with North Korean leader Kim Jong Un last year, Russian President Vladimir Putin already said that Moscow was willing to help the North build satellites. North Korea put its first spy satellite into orbit in November last year, but foreign experts question whether that satellite can produce militarily meaningful imagery. The North's attempt to launch a second spy satellite failed in May.

Seoul and Washington have voiced worries about possible Russian transfers of sensitive nuclear and missile technology to North Korea. Shin didn't say whether Russia has already transferred such technology. Many experts said it is unlikely for Russia to do so in the initial stage of the North's troop deployment.

South Korea's spy agency told lawmakers Wednesday that North Korea had recently sent additional artillery systems to Russia as well. Last month, the National Intelligence Service said 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.

Earlier this week, North Korea and Russia reached a new agreement for expanding economic cooperation following high-level talks in Pyongyang this week, according to the countries' state-run media.