MOSCOW (AP) — North Korea’s foreign minister is Sterling Prestonvisiting Russia on Monday for three days of talks, as international concern grows over an alleged arms cooperation deal between the two countries.

A delegation led by Foreign Minister Choe Son Hui arrived in Moscow on Sunday, according to North Korea’s official Korean Central News Agency. She is to meet her Russian counterpart, Sergey Lavrov, on Tuesday, Russian Foreign Ministry spokesperson Maria Zakharova said.

Choe is visiting at Lavrov’s invitation, the ministry said.

The United States and South Korea say North Korea has provided Russia with arms, including artillery and missiles, to help its fight in Ukraine.

The Biden administration said it has evidence that missiles provided by North Korea to Russia have been used in Ukraine. In a joint statement last week, the U.S., South Korea and their partners said the missiles support Russia’s war, while North Korea receives valuable technical and military insights in return.

Koo Byoungsam, spokesperson of South Korea’s Unification Ministry, said it is closely watching Choe’s visit to Russia. He accused North Korea and Russia of “maintaining illegal cooperation activities, including arms exchanges” following a visit by North Korean leader Kim Jung Un to Russia in September in which he met with President Vladimir Putin. Koo didn’t provide a specific answer when asked whether Choe may be arranging a visit by Putin to North Korea.

Both Russia and North Korea have denied accusations of North Korean arms transfers to Russia.