A new agreement between Russia and North Korea reached by their leaders requires the countries to use all available means to provide immediate military assistance in the event of war, North Korean state media said.

The North’s official Korean Central News Agency on Thursday reported the language of the comprehensive strategic partnership agreement reached by its leader Kim Jong Un and Russian President Vladimir Putin in Pyongyang on Wednesday. The agency said Article 4 of the agreement states that if one of the countries gets invaded and is pushed into a state of war, the other must deploy “all means at its disposal without delay” to provide “military and other assistance.”

The deal could mark the strongest connection between Moscow and Pyongyang since the end of the Cold War.

Both Kim and Putin described it as a major upgrade of their relations, covering security, trade, investment, cultural and humanitarian ties.

The summit came as the U.S. and its allies expressed growing concerns over a possible arms arrangement in which Pyongyang provides Moscow with badly needed munitions for its war in Ukraine, in exchange for economic assistance and technology transfers that could enhance the threat posed by Kim’s nuclear weapons and missile program.

Following their summit, Kim said the two countries had a “fiery friendship,” and that the deal was their “strongest-ever treaty,” putting the relationship at the level of an alliance.

He vowed full support for Russia’s war in Ukraine.

Putin called it a “breakthrough document” reflecting shared desires to move relations to a higher level.

North Korea and the former Soviet Union signed a treaty in 1961, which experts say necessitated Moscow’s military intervention if the North came under attack.

The deal was discarded after the collapse of the USSR, replaced by one in 2000 that offered weaker security assurances.

South Korean officials said they were still interpreting the results of the summit, including what Russia’s response might be if the North comes under attack, and whether the new deal promises a similar level of protection with the 1961 treaty.

South Korean officials didn’t immediately comment on the North Korean report about the details of the deal.

Tensions on the Korean Peninsula are at their highest point in years, with the pace of both Kim’s weapons tests and combined military exercises involving the U.S., South Korea and Japan intensifying in a tit-for-tat cycle.

The Koreas also have engaged in Cold War-style psychological warfare that involved North Korea dropping tons of trash on the South with balloons, and the South broadcasting anti-North Korean propaganda with its loudspeakers.