The United States and South Korea have accused the nuclear-armed North of sending more than 10,000 soldiers to help Russia fight Ukraine, with experts saying Kim Jong Un was eager to gain advanced technology, and battle experience for his troops, in return.

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".

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, the report said, quoting British Foreign Secretary David Lammy saying that the oil was payment for weapons and troops Pyongyang had sent to Moscow.

- 'Breakthrough document'

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

It obligates both states to provide military assistance "without delay" in the case of an attack on the other and to cooperate internationally to oppose Western sanctions.

Putin hailed the deal as a "breakthrough document".

Experts say Pyongyang could be using Ukraine as a means of realigning foreign policy.

By sending soldiers, North Korea is positioning itself within the Russian war economy as a supplier of weapons, military support and labour -- potentially bypassing its traditional ally, neighbour and main trading partner, China, according to analysts.

Russia can also give North Korea access to its vast natural resources, such as oil and gas, they say.

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 and Russia are under rafts of UN sanctions -- Kim for his nuclear weapons programme, and Moscow for the Ukraine war.

When asked publicly about the deployment of North Korean troops last month, Putin deflected the question to criticise the West's support of Ukraine.

North Korea said last month that any troop deployment to Russia would be "an act conforming with the regulations of international law", but stopped short of confirming that it had sent soldiers.

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.