Train carrying Kim Jong Un enters Russia en route to meeting with Vladimir Putin

LONDON -- The bullet-proof train carrying Kim Jong Un crossed the border into Russia as the North Korean leader made his way to a summit with President Vladimir Putin, local media reported.

Russian media posted a video of the green-and-gold train traveling north in Primorye, the far-east district where Vladivostok is located.

The green-and-gold train in the video appeared to match images released on Monday by the Korean Central News Agency, North Korea's state-run media.

The Kremlin on Monday announced that Putin would travel to Vladivostok for an economic forum. The office said the two leaders would meet in the coming days, but didn't offer specifics about the timing or location of the summit.

U.S. officials said discussions have advanced between the two countries about a possible plan for North Korea to supply weapons to Russia.

Kim's train departed Pyongyang on Sunday, beginning its journey to Russia for a planned meeting with President Vladimir Putin, a South Korean official told ABC News.

"Our Dearest Comrade Kim Jong Un will be visiting Russia upon invitation from Russian President Vladimir Putin. During the visit he will meet Putin and hold a summit," the Korean Central News Agency reported Monday.

South Korean officials said the train was expected to cover the about 683 miles to Vladivostok in about 20 hours, a timeline that included with a long stopover at the Russia-North Korea border, needed to switch to wheels that would fit Russian railway tracks.

ABC News' Ellie Kaufman and Joohee Cho contributed to this story.