BEIJING (Kyodo) -- North Korean Foreign Minister Choe Son Hui has left for an official visit to Russia, the North's state media said Tuesday, amid growing concern about the transfer of North Korean troops to Russia for potential deployment to Ukraine.

Choe departed from Pyongyang by plane on Monday, the official Korean Central News Agency reported without elaborating on the purpose and schedule of the visit. She last visited Russia in September to attend an international conference in St. Petersburg.

There is speculation that Choe may discuss with Russian officials possible talks in Moscow between North Korean leader Kim Jong Un and Russian President Vladimir Putin.

The Russian Embassy in Pyongyang said in a Facebook post Choe's visit is part of the countries' dialogue under the comprehensive strategic partnership treaty signed by Kim and Putin in Pyongyang in June.

The treaty includes a clause covering mutual assistance in case of aggression against either country.

North Korea is believed to have sent about 10,000 troops to eastern Russia for training and they are likely to augment Moscow's forces "near Ukraine over the next several weeks," according to the U.S. Defense Department. The United States also believes some of the soldiers have already moved closer to Russia's western Kursk region.