Russia will provide North Korea with warplanes in exchange for thousands of soldiers Pyongyang has reportedly sent to support Moscow’s war against Ukraine, the head of the U.S. Pacific Forces said.

Admiral Samuel Paparo, commander of the Indo-Pacific Command, told reporters that Kim Jong Un’s regime will receive MiG-29 and Su-27 aircraft, Soviet-era fighter jets introduced four decades ago, according to Aviation Week Network.

The news follows reports confirmed by the Pentagon that thousands of North Korean troops were stationed in Russia’s front-line Kursk region, where Ukrainian forces have been launching a counteroffensive since August. Paparo said that although these soldiers were in combat zones, they had not yet taken part in combat – a statement that contradicted recent comments by Ukrainian President Volodymyr Zelensky.

Paparo said that while the MiG-29 and Su-27 are not modern aircraft, they are still “impressive” aircraft that could bolster North Korea’s air power. Pyongyang’s fighter jet fleet was purchased by the Soviet Union and later by China.

The communist country’s bitter rival South Korea operates upgraded versions of the U.S.-made F-15 and F-16 fighters, F-35 stealth fighters and fighter jets such as the KAI T-50.

The United States and its allies have warned that these growing military ties could lead to dangerous transfers of advanced technology in support of Pyongyang’s United Nations-approved ballistic missile and nuclear weapons programs.

Last month, South Korea’s defense chief said Russia had supplied air defense missiles to North Korea in return for Pyongyang’s deployment of new troops.

Seoul has also accused Pyongyang of shipping thousands of containers filled with ammunition to replenish depleted Russian supplies.

Ukraine continues to operate aging F-16s supplied by European allies and is expected to receive additional batches next year. France has also pledged Mirage 2000, fighter jets that, like the MiG-29 and Su-27, entered service about 40 years ago.

The increasing rapprochement between Russia and North Korea sparked concern in the region. US Defense Secretary Lloyd Austin called the North Korean ground forces a “dangerous and destabilizing escalation.”

This summer, Russian President Vladimir Putin and his North Korean counterpart Kim Jong Un signed a landmark military cooperation agreement. The allies stressed that only countries planning an attack need to be concerned about the pact and denied that North Korean troops were on the ground.

Newsweek reached out to the Russian Foreign Ministry and the North Korean Embassy in Beijing, China, via email outside normal office hours for comment.