Moscow, Russia - Russia provided North Korea with oil, anti-air missiles and economic help in exchange for troops to support Moscow's war on Ukraine, South Korean officials and a research group said Friday.

The US and South Korea have accused the nuclear-armed North of sending more than 10,000 soldiers to help Russia fight Ukraine.

Asked what Seoul believes the North has received for the deployment, South Korea's top security advisor Shin Won-sik said: "It has been identified that equipment and anti-aircraft missiles aimed at reinforcing Pyongyang's vulnerable air defense system have been delivered to North Korea."

Speaking to local broadcaster SBS, Shin added that North Korea has received "various forms of economic support."

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Separately, non-profit research group Open Source Centre said Pyongyang obtained oil shipments from Russia.

It cited satellite images showing that more than a dozen North Korean oil tankers making a total of 43 trips to an oil terminal in Russia's ports over the last eight months, according to a BBC report. Pictures also showed empty tanks leaving almost full after arrival.

Kim and Russian President Vladimir Putin signed a strategic partnership treaty in June, during the Kremlin chief's visit.

It commits both states to providing military assistance "without delay" in the case of an attack on the other and to cooperating internationally to oppose Western sanctions.

North Korean Foreign Minister Choe Son Hui recently visited Moscow and said her country would "stand firmly by our Russian comrades until victory day."

She called Moscow's offensive against Ukraine a "sacred struggle" and said Pyongyang believed in Putin's "wise leadership".

North Korea's deployment of troops has led to a shift in tone from Seoul, which has resisted calls to send lethal weapons to Kyiv, but recently indicated it might change its longstanding policy.