DPRK leader Kim Jong Un steps out of his train during a visit to Russia in 2019. (AP)

DPRK leader Kim Jong Un is said to have boarded the train on his way to Russia, according to South Korea's Yonhap news agency on Monday, citing an unidentified official.

Kim's train "appears to have departed," Yonhap said, as speculations arise that the leader who rarely leaves his country and has not traveled since the pandemic, is due to meet Russian President Vladimir Putin to allegedly negotiate arms deals.

The last time Kim traveled to Russia to meet Putin was in 2019 when he made a train journey to Vladivostok, Russia's Far East.

The New York Times was recently notified by US officials and other sources that Kim is possibly expected to board the armored train to Vladivostok, which is currently hosting the Eastern Economic Forum (EEF) from September 10 to 13.

Back in July, Putin praised the DPRK's "firm support for special military operations against Ukraine".

Russia has been a supporter and ally of the DPRK for decades, which goes back to the founding of the DPRK 75 years ago, which was commemorated last week.

This comes after DPRK Ambassador to Moscow Sin Hong Chol stated on Thursday that the DPRK will increase military coordination with Russia in its fight against "a common enemy".

The diplomat emphasized the DPRK's confidence in the strength of its relations with Russia with which each shares "a deep history and tradition of friendship and cooperation."