The upper house of Russian parliament on Wednesday ratified a treaty with North Korea envisioning mutual military assistance, a move that comes as the US has confirmed the deployment of 10,000 North Korean troops to Russia.

The “comprehensive strategic partnership treaty,” which Russian President Vladimir Putin signed with North Korean leader Kim Jong Un on a visit to Pyongyang in June, obliges Russia and North Korea to immediately provide military assistance using “all means” if either is attacked. It marked the strongest link between Moscow and Pyongyang since the end of the Cold War.

The pact’s endorsement by the upper house, the Federation Council, follows its ratification by the lower house, the State Duma, on Oct 24.

On Monday, US State Department spokesperson Matthew Miller told reporters that up to 10,000 North Korean soldiers were in Russia’s Kursk region and were preparing to join Moscow’s fight against Ukraine in the coming days. If they engage in combat, it would be North Korea’s first participation in a large-scale conflict since the end of the 1950-53 Korean War.

Moscow and Pyongyang have responded vaguely to South Korean and Western claims of the North Korean troop deployment to Russia, emphasizing that their military cooperation conforms with international law, without directly admitting the presence of the North’s forces in Russia.

On Monday, Putin met with North Korean Foreign Minister Choe Son Hui, who was on an official visit to Russia since late last week. During her talks with Russian Foreign Minister Sergey Lavrov, Choe reiterated Pyongyang’s support “for the just fight of Russia’s military and people to defend their country’s sovereign rights and security interests” in Ukraine.

Russia and North Korea have previously rejected assertions by the US and its allies that Pyongyang has given Moscow ballistic missiles and millions of artillery shells for use in Ukraine.

South Korean and Western officials voiced concerns about Russia’s possible transfer of technology to North Korea to enhance its nuclear and missile programmes in exchange for its troops.

On Tuesday, North Korea fired a barrage of short-range ballistic missiles into the sea in a demonstration of its military capability hours before the US presidential election.