The United States has expressed growing concern over North Korea’s increasing military support for Russia in the 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 increasing evidence that North Korea is supplying materials such as artillery and missiles to support Moscow.

Campbell spoke to reporters after talks with South Korean and Japanese officials about renewing international pressure on North Korea. His comments came as tensions remain high on the Korean peninsula and Pyongyang is stepping up its weapons tests and threats against South Korea.

North Korea’s increasing military support for Russia is a cause for concern for the United States and its allies.

Campbell addressed Ukrainian President Volodymyr Zelensky’s claims that Pyongyang was sending soldiers to fight in Ukraine, saying the U.S. 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 missile attack on October 3, raising questions about Pyongyang’s possible deployment of personnel to the front.

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

Russia’s relations with North Korea have increased since the Ukraine War began on February 24, 2022, particularly given Moscow’s increasing international isolation. In March, Russia vetoed a United Nations Security Council resolution, effectively ending international monitoring of sanctions against North Korea.

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

On Wednesday, the United States, South Korea, Japan and eight other Western governments announced plans to create a multinational team to oversee North Korean sanctions enforcement. The move is aimed at countering North Korea’s arms sales to Russia, which Campbell said 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 Russia-Ukraine War also comes at a time when North Korea continues to pose a threat to South Korea. Pyongyang has accused Seoul of using drones to drop propaganda leaflets over its territory and threatened retaliation if such actions continued.

In a symbolic move, North Korea on Tuesday demolished parts of unused road and railway lines that once linked it to South Korea. The demonstration, aimed at expressing frustration with the south’s conservative government, underscores rising tensions on the peninsula.

Since 2022, North Korean leader Kim Jong Un has used the distraction of Russia’s war in Ukraine to speed up weapons tests. In response, Washington, Seoul and Tokyo have increased their joint military exercises and strengthened their nuclear deterrence strategies.

Following 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 common stance between South Korea and the United States to respond decisively to North Korean provocations,” Kim said.

This article contains reporting from The Associated Press.