South Korean and Western leaders have expressed concern that North Korea’s involvement could help prolong Russian aggression in Ukraine and that Russia could offer technology in exchange that could increase the threat posed by North Korea’s nuclear weapons and missile program.

Pentagon spokeswoman Sabrina Singh told reporters Monday that some North Korean troops have already moved closer to Ukraine and are likely heading toward the Kursk border region, where Russia is struggling to repel a Ukrainian incursion.

South Korean President Yoon Suk Yeol, in phone calls with European Union Commission President Ursula von der Leyen and NATO Secretary General Mark Rutte, shared the services’ assessments on Monday South Korean intelligence reports that North Korean troops could be deployed to the battle fronts “more quickly than expected”. He called for closer coordination with European governments aimed at “monitoring and blocking” illegal trade between Pyongyang and Moscow, Yoon’s office said in a statement.

After initially denying allegations about the deployment of North Korean troops, Pyongyang and Moscow have taken a vaguer stance, saying their military cooperation is consistent with international law, without directly admitting the presence of North Korean forces in Russia.

North Korea has also been accused of supplying millions of artillery shells and other military equipment to Russia to fuel its war in Ukraine. The United States and its partners have called Russia’s purchase of North Korean personnel and supplies a violation of U.N. Security Council resolutions and raised suspicions that Moscow is helping Pyongyang evade sanctions and to illegally finance its arms program.

“Illegal military collusion between Russia and North Korea poses a significant threat to the security of the international community and a serious problem that could potentially harm our security. We need to carefully consider all possibilities and prepare countermeasures,” Yoon said Tuesday at a Cabinet meeting in Seoul.

Yoon raised the possibility of supplying weapons to Ukraine last week while saying Seoul was preparing countermeasures that could be deployed in stages depending on the degree of military cooperation between Pyongyang and Moscow.

South Korea, a growing arms exporter, has provided humanitarian aid and other non-lethal support to Ukraine and joined U.S.-led economic sanctions against Moscow. It has so far resisted calls from kyiv and NATO to directly supply weapons to Ukraine, citing a long-standing policy of not supplying weapons to countries engaged in active conflict.