South Korean and Western leaders have expressed concern that North Korea’s intervention could help prolong Russia’s aggression in Ukraine, and that Russia could in turn offer technology that could advance the threat posed by North Korea’s nuclear weapons and missile program.

Pentagon spokeswoman Sabrina Singh told reporters on Monday that some North Korean troops are approaching Ukraine and are thought to be moving toward the Kursk border region, where Russia is trying to push back the Ukrainian attack.

In phone calls with European Union Commission President Ursula von der Leyen and NATO Secretary General Mark Rutte on Monday, South Korean President Yoon Suk Yeol shared South Korean intelligence assessments that North Korean troops could be deployed to battle fronts “faster than expected.” A statement from Yoon’s office said Yoon called for closer coordination with European governments, aiming to “monitor and prevent” illegal exchanges between Pyongyang and Moscow.

After initially denying allegations about North Korea’s troop deployment, Pyongyang and Moscow took a more ambiguous stance, arguing that their military cooperation complies with international law without directly acknowledging the presence of North Korean forces in Russia.

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

“Illegal military collusion between Russia and North Korea is a serious issue that poses a significant security threat to the international community and has the potential to harm our security. “We must thoroughly examine all possibilities and prepare countermeasures,” Yoon said at the Cabinet meeting in Seoul on Tuesday .

While Yoon raised the possibility of supplying weapons to Ukraine last week, he said Seoul was preparing countermeasures that could be implemented in phases depending on the degree of military cooperation between Pyongyang and Moscow.

South Korea, a growing arms exporter, has provided humanitarian aid and other nonlethal support to Ukraine and participated in U.S. economic sanctions against Moscow. It has so far resisted calls from Kiev and NATO to provide arms directly to Ukraine, citing its long-standing policy of not providing arms to countries in active conflict.