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 Pyongyang has received for the troops, top security advisor Shin Won-sik said: "It has been identified that equipment and anti-aircraft missiles aimed at reinforcing Pyonyang'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" and "following the failure (launch) on May 27, North Korea has been working on satellite-related technology".

Experts have previously said that in return for the troops, North Korea was likely aiming to acquire military technology, ranging from surveillance satellites to submarines, as well as possible security guarantees from Moscow.

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 labor -- potentially bypassing its traditional ally, neighbour and main trading partner, China, according to analysts.

Russia can also provide 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 program, 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.