SEOUL, South Korea — US President Trump became the first sitting American commander in chief to set foot in North Korea on Sunday as he greeted Kim Jong-un, the country’s leader, at the heavily fortified Demilitarized Zone and the two agreed to send their negotiators back to the table to seek a long-elusive nuclear agreement.

Met in the middle by a beaming Mr. Kim, Mr. Trump stepped across a low concrete marker at 3:46 p.m. local time and walked 20 steps to the base of a building on the North Korean side for an encounter carried live on international television — an unprecedented, camera-friendly demonstration of friendship intended to revitalize stalled talks.

"It is good to see you again,” a seemingly exuberant Mr. Kim told the president through an interpreter. "I never expected to meet you in this place.”

"Big moment, big moment,” Mr. Trump told him.

After about a minute on officially hostile territory, Mr. Trump escorted Mr. Kim back over the line into South Korea, where the two briefly addressed a scrum of journalists before slipping inside the building known as Freedom House for a private conversation. Mr. Trump said he would invite Mr. Kim to visit him at the White House.

"This has a lot of significance because it means that we want to bring an end to the unpleasant past and try to create a new future, so it’s a very courageous and determined act,” Mr. Kim told reporters.

"Stepping across that line was a great honor,” Mr. Trump replied. "A lot of progress has been made, a lot of friendships have been made and this has been in particular a great friendship.”

A showman by nature and past profession, Mr. Trump delighted in the drama of the moment. Never before had American and North Korean leaders gotten together at the line bristling with concertina wire and weapons, where heavily armed forces have faced off across a tense divide for 66 years since the end of fighting in the Korean War.

The encounter in Panmunjom was cast as a brief greeting, not a formal negotiation, but the two ended up together for a little more than an hour. After emerging, Mr. Trump said he and Mr. Kim had agreed to designate negotiators to resume conversations in the next few weeks. The American team will continue to be headed by Stephen Biegun, the special envoy, but it remained unclear who would be on the North Korean side of the table.

Agencies