North Korean leader Kim Jong Un's train "appears" to have left for Russia, South Korea's Yonhap news agency reported Monday, citing an unidentified official.

Kim's train "appears to have departed," Yonhap reported.

Speculation has been mounting that Kim, who rarely leaves his country and has not travelled since the coronavirus pandemic started, will meet Russia's President Vladimir Putin to discuss arms deals.

US and other officials recently told The New York Times that Kim is likely to head by armoured train to Vladivostok, on Russia's Pacific coast not far from North Korea, this month to meet Putin.

Russia, a historical ally of Pyongyang, was a crucial backer of the isolated country for decades and their ties go back to the founding of North Korea 75 years ago.

Kim has been steadfast in his support for Moscow's invasion of Ukraine, including, Washington says, supplying rockets and missiles. 

In July, Putin hailed Pyongyang's "firm support for special military operations against Ukraine".