The formalization of the treaty comes as the United States and South Korea have accused the nuclear-armed North of sending more than 10,000 soldiers to help Russia fight Ukraine.

Experts say North Korean leader Kim Jong Un is keen to acquire advanced technology from Moscow and battle experience for his troops in return.

Kim and Russian President Vladimir Putin signed the strategic partnership deal during the Kremlin chief's visit to Pyongyang.

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

It came into effect from Wednesday, when the ratification documents were exchanged in Moscow by the countries' vice foreign ministers Kim Jong Gyu and Andrei Rudenko, KCNA reported.

Lawmakers in Moscow last month voted unanimously for the deal and it was later signed by Putin. Pyongyang said it was ratified by a decree from Kim.

The treaty will serve "as a strong driving force accelerating the establishment of an independent and just multi-polarized world order without domination, subjugation and hegemony," KCNA said.

Analysts have suggested Pyongyang could be using Ukraine as a means of realigning its 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 even bypassing traditional ally, neighbour and main trading partner China, they say.

North Korea and Russia have strengthened their military ties since Moscow's invasion of Ukraine in February 2022.

Both countries are under rafts of UN sanctions -- the former for its nuclear weapons programme and the latter for the Ukraine conflict.

Kim said last week during a visit to Pyongyang by Russian Defence Minister Andrei Belousov that his government, army and people would "invariably support the policy of the Russian Federation to defend its sovereignty and territorial integrity".

Putin hailed the deal in June as a "breakthrough document".