North Korea has ratified a mutual defence treaty with Russia signed by the two countries’ leaders in June, which calls for each side to come to the other’s aid in case of an armed attack, state media KCNA said on Tuesday.

The report came amid international criticism over increasing military cooperation between the two countries, with North Korea having sent tens of thousands of troops to Russia to support its war against Ukraine.

North Korean leader Kim Jong Un signed a decree to ratify the pact on Monday, KCNA said, adding it takes effect when both sides exchange the ratification instruments.

Russian President Vladimir Putin has also signed the treaty into law, which stipulates that the two countries should “immediately provide military and other assistance using all available means” if either side is in a state of war.

Kim clinched the accord with Putin at a summit in June, touting it as a step to elevate bilateral ties to something akin to an “alliance”.

Seoul, Washington and Kyiv have said there are more than 10,000 North Korean soldiers in Russia, and U.S. officials and Ukraine’s defence minister said some of them have engaged in combat in Kursk, near the Ukraine border.

Ukrainian President Volodymyr Zelenskiy said last week that North Korean troops had suffered casualties in combat with his country’s forces, and the first battles between them “open a new page in instability in the world.”