Ukraine and its allies say some North Korean soldiers are already deployed to fight in the Russian region of Kursk.

The upper house of Russia’s parliament has ratified a landmark mutual defence pact with North Korea as top Ukrainian officials report clashes with Pyongyang’s soldiers on the front lines.

Lawmakers in the Federation Council of Russia, which acts as a senate, approved the bill on ratification of the Treaty of Comprehensive Strategic Partnerships between the two countries on Wednesday, state media reported.

The treaty was signed in Pyongyang on June 19 during a lavish state visit by Russian President Vladimir Putin, who is deepening ties with North Korea as the war with Ukraine approaches its third anniversary.

The unanimous vote in the upper house formalises months of increasing security cooperation between the two nations, the largest since the time they were Communist allies during the Cold War.

Lawmakers in the lower house of parliament, the State Duma, had ratified the legislation on October 24.

The pact obliges Russia and North Korea to immediately provide military assistance using “all means” if there is any “aggression” against either country.

The parliamentary approval comes as Ukrainian Defence Minister Rustem Umerov told the South Korean broadcaster KBS that the Ukrainian military had its first confrontation with North Korean soldiers. He said a “small group” of North Korean soldiers were attacked.

Umerov said he expects Ukrainian soldiers to face more North Korean forces in combat but there have been only small contacts so far and most North Korean soldiers are still undergoing training before being deployed.

Ukrainian President Volodymyr Zelenskyy, who had earlier condemned the West’s lack of response to the arrival of the North Korean soldiers on the front lines, said these “first battles with North Korea open a new chapter of instability in the world”.

Russia has deployed the North Korean soldiers to its own border region of Kursk, which has seen an offensive by the Ukrainian military since August in an effort by Kiev to chip away at Russian advances on Ukrainian soil.

Ukraine estimated 11,000 North Korean soldiers were in Kursk.

Moscow and Pyongyang have so far not directly commented on the presence of North Korean soldiers on the front lines, but the deployment has been discussed by NATO, the United States and South Korea.

South Korea has also threatened that it could send arms to Ukraine if North Korean soldiers are not withdrawn.

Observers said Pyongyang could have been offered payments or deliveries of Russian military technology in exchange for the troops.