Kim Jong Un crosses DMZ line for historic meeting with South Korea

North Korean leader Kim Jong Un has crossed the line dividing the demilitarized zone to meet with South Korean President Moon Jae-in in what's being described as a historic summit.

Kim made that first step, a great leap for the Korean Peninsula. The two posed for photos facing the North, then facing the South. Just about when Moon ushered Kim to walk toward the red carpet, Kim made a gesture offering Moon to take a step back across the demarcation line, to his side of the border. Whether pre-planned or not, the two smiled and took that step holding hands.

Escorted by traditional music, Moon and Kim walked together to the "Peace House," a three-story building where the official summit will take place.

Inside the "Peace House," Kim signed the guest book, writing in it for more than a minute. The two leaders then went into a reception room for a private conversation.

It's the first time since 2007 leaders of the two countries have met and is part of a recent thawing of relations as South Korea and the United States have focused on diplomacy in their efforts to dismantle North Korea's nuclear program.

The summit has been in the works for some time, with the stage being set by two previous meetings between high-level officials from the North and South, as well as the North's participation in the Winter Olympics in Pyeongchang, South Korea, earlier this year. The two nations marched under one flag.

The meeting is also a precursor to President Donald Trump's own announced meeting with Kim, which is expected to take place in May or early June, though Trump cast doubt on it Thursday.

"It could be that I walk out quickly -- with respect, but it could be. It could be that maybe the meeting doesn't even take place. Who knows?" Trump told Fox News' "Fox & Friends."

The U.S. and North Korea have narrowed down the location to five possibilities, according to Trump, with three or four dates in the running. A senior U.S. official previously told ABC News that Trump had ruled out China and that it was highly unlikely Kim would agree to meet in the U.S. or that Trump would agree a location in North Korea.

Possible venues are Europe -- including Switzerland, where Kim attended a university, or Sweden, the U.S.'s protecting power in North Korea -- South Asia, and the DMZ between North and South Korea.

ABC News' Conor Finnegan contributed to this report.