The United States has expressed growing concern over North Korea’s increasing military support for Russia in its ongoing war against Ukraine.

Deputy Secretary of State Kurt Campbell said Wednesday that Washington and its allies are alarmed by Pyongyang’s nuclear threats and growing evidence that North Korea is providing materials, such as artillery and missiles, to help Moscow.

Campbell spoke to reporters after talks with South Korean and Japanese officials about renewed international pressure on North Korea. His comments came as tensions remained high on the Korean Peninsula and Pyongyang stepped up weapons tests and threats against South Korea.

North Korea’s growing military support for Russia has become a concern for the US and its allies.

Addressing claims by Ukrainian President Volodymyr Zelensky that Pyongyang is sending soldiers to fight in Ukraine, Campbell said the US could not confirm the reports. However, he stressed that Washington is “closely monitoring the situation” and remains concerned about North Korea’s involvement.

Ukrainian media reported that six North Korean soldiers were killed in a rocket attack on October 3, raising questions about Pyongyang’s possible deployment of frontline personnel.

The reports followed claims by Ukrainian officials that North Korean troops deserted shortly after arriving in Ukraine, indicating low morale or mismanagement.

Russian ties with North Korea have grown since the start of the war in Ukraine on February 24, 2022, especially as Moscow faces increasing international isolation. In March, Russia vetoed a United Nations Security Council resolution, effectively ending international oversight of sanctions against North Korea.

Western countries have accused Moscow of trying to shield its arms purchases from Pyongyang to continue its attacks in Ukraine.

On Wednesday, the US, South Korea, Japan and eight other Western governments announced plans to launch a multinational team to oversee enforcement of North Korean sanctions. The move is aimed at countering North Korea’s arms sales to Russia, which Campbell says are contributing to instability in Europe.

“We are concerned about them and (…) we have agreed that we will continue to monitor the situation closely,” Campbell said.

North Korea’s involvement in the war between Russia and Ukraine also stems from the fact that the country continues to threaten South Korea. Pyongyang has accused Seoul of using drones to distribute propaganda leaflets across its territory and threatened to retaliate if such actions continue.

In a symbolic move, North Korea on Tuesday demolished parts of unused road and rail lines that once connected the country to South Korea. The demonstration, aimed at demonstrating frustration with the South’s conservative government, underlines heightened tensions on the peninsula.

Since 2022, North Korean leader Kim Jong Un has taken advantage of the distraction caused by Russia’s war in Ukraine to accelerate weapons testing. In response, Washington, Seoul and Tokyo have stepped up their joint military exercises and strengthened their nuclear deterrent strategies.

After the talks with Campbell and Japanese Vice Foreign Minister Masataka Okano, South Korean Vice Foreign Minister Kim Hong Kyun condemned North Korea’s provocations.

“We agreed to maintain a solid combined posture of South Korea and the US to respond strongly to North Korea’s provocations,” Kim said.

This article contains reporting from The Associated Press.