WASHINGTON -- U.S. officials worked with North Korea over the weekend on plans for the on-off-on meeting between President Donald Trump and North Korean leader Kim Jong Un, as Trump continued to signal optimism for the sit-down.

Trump tweeted Sunday that a U.S. team was in North Korea to work on the possible summit. Another delegation was in Singapore to discuss logistics. On Monday, the White House said Trump had spoken with Japanese Prime Minister Shinzo Abe about the North Korea efforts, adding that the two planned to meet before the "expected meeting" with Kim to co-ordinate strategy.

Trump withdrew from the planned June 12 summit in Singapore last Thursday, but quickly announced it could still happen. The president's Sunday tweet offered praise for the longtime U.S. adversary, another signal that his concerns about the North's stance toward the summit had been allayed.

"Our United States team has arrived in North Korea to make arrangements for the Summit between Kim Jong Un and myself," he tweeted Sunday. "I truly believe North Korea has brilliant potential and will be a great economic and financial Nation one day. Kim Jong Un agrees with me on this. It will happen!"

Both the State Department and South Korea's Foreign Ministry said the American and North Korean officials have been engaged in talks at the Korean village of Panmunjom, which straddles the border inside the Demilitarized Zone, or DMZ. The border can be crossed simply by stepping across a painted line, but moving beyond several footsteps into the North at Panmunjom would be rare for U.S. officials.

South Korean President Moon Jae-in gave details about his surprise meeting Saturday with Kim in Panmunjom, saying Kim had committed to sitting down with Trump and to a "complete denuclearization of the Korean Peninsula."

U.S. Secretary of State Mike Pompeo tapped veteran American diplomat Sung Kim, the U.S. ambassador to the Philippines, to handle pre-summit negotiations. On a separate but complementary track was the CIA team Pompeo set up last year when he headed the spy agency. And on a third track was a White House logistical group sent to Singapore on Sunday to prepare in case the summit takes place. It was led by Joe Hagin, White House deputy chief of staff for operations.

The Korean leaders' second summit in a month saw bear hugs and broad smiles. But their quickly arranged meeting appeared to highlight a sense of urgency on both sides of the world's most heavily armed border.

The talks, which Moon said Kim requested, capped a whirlwind 24 hours of diplomatic back and forth. They allowed Moon to push for a U.S.-North Korean summit, which he sees as the best way to ease the animosity that had some fearing a war last year.

Kim may see a meeting with Trump as necessary to easing pressure from crushing sanctions and to winning security assurances in a region surrounded by enemies.

Moon told reporters that Kim said he's willing to co-operate to end confrontation and work toward peace for the sake of the successful summit with Trump.

Moon said he told Kim that Trump has a "firm resolve" to end hostile relations with North Korea and initiate economic co-operation if Kim implements "complete denuclearization."

"What Kim is unclear about is that he has concerns about whether his country can surely trust the United States over its promise to end hostile relations (with North Korea) and provide a security guarantee if they do denuclearization," Moon said.

"During the South Korea-U.S. summit, President Trump said the U.S. is willing to clearly put an end to hostile relations (between the U.S. and North Korea) and help (the North) achieve economic prosperity if North Korea conducts denuclearization," he said.

Kim, in a dispatch issued by the North's state-run news service earlier Sunday, "expressed his fixed will" on a North Korea-U.S. summit.

During Saturday's inter-Korean summit, the Korean leaders agreed to "positively co-operate with each other as ever to improve (North Korea)-U.S. relations and establish (a) mechanism for permanent and durable peace."

They agreed to have their top officials meet again June 1. Moon said military generals and Red Cross officials from the Koreas will also meet separately to discuss how to ease military tensions and resume reunions of families separated by the 1950-53 Korean War.