President Donald Trump has said he will invite North Korean leader Kim Jong-un to the White House if their summit next week goes well.

Speaking on the White House lawn after an afternoon of discussions with Japanese Prime Minister Shinzo Abe, Mr Trump said he would “certainly” invite Mr Kim to the White House if talks about Pyongyang's nuclear programme were productive. He added that he thought the North Korean leader would look on the invitation “very favourably”.

The comments came less than a week after Mr Trump decided to re-engage with the Singapore summit, which he abruptly called off at the end of May blaming "hostile" comments from North Korea. He warned on Thursday that he could easily walk away from the talks again, saying: “I did it once before!”

But Mr Trump seemed optimistic at the White House press conference, saying it was possible that the two countries could sign an agreement ending the Korean War. Signing a joint communique would be “the easy part” of a long process of negotiations, Mr Trump said.

Earlier, he told reporters that he felt “well prepared” for the summit, adding: “I don’t think I have to prepare that much.”

After the press conference with Mr Abe and Mr Trump, US Secretary of State Mike Pompeo said at the White House press briefing that the president had been preparing for months for next week's summit in Singapore.

“So over months and months, days and days, President Trump has been receiving briefings on this issue about the military aspects, commercial, economic aspects of it, the history of the relationship and in the past few months near daily briefings including today where we have been providing the president all of the information that he needs,” Mr Pompeo said.

North Korea Military Parade Show all 6 1 /6 North Korea Military Parade North Korea Military Parade In this image made from video released by KRT on Feb. 8, 2018, a military parade is held at Kim Il Sung Square in Pyongyang, Thursday, Feb. 8 2018, just one day before South Korea holds the opening ceremony for the Pyeongchang Winter Olympics. (KRT via AP Photo) AP North Korea Military Parade This screen grab taken from North Korea's KCTV on February 8, 2018 shows members of North Korea's military taking part in a parade, with missiles being displayed, in Kim Il Sung Square in Pyongyang. North Korea staged a military parade in Pyongyang on February 8 to mark the 70th anniversary of its armed forces, in a show of strength just a day before the Pyeongchang 2018 Winter Olympic Games open in the South. / AFP PHOTO / KCTV / KCTV / -----EDITORS NOTE --- RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / KCTV" - NO MARKETING - NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS - NO ARCHIVESKCTV/AFP/Getty Images AFP/Getty Images North Korea Military Parade In this image made from video released by KRT, North Korean soldiers march during a military parade in Pyongyang, Thursday, Feb. 8 2018. North Korea held a military parade and rally on Kim Il Sung Square on Thursday, just one day before South Korea holds the opening ceremony for the Pyeongchang Winter Olympics. (KRT via AP Photo) AP North Korea Military Parade In this image made from video released by KRT on Feb. 8, 2018, North Korean leader Kim Jong Un, right, inspects honor guards, along with his wife Ri Sol Ju, center, during a military parade in Pyongyang, Thursday, Feb. 8 2018. North Korea held a military parade and rally on Kim Il Sung Square on Thursday, just one day before South Korea holds the opening ceremony for the Pyeongchang Winter Olympics. (KRT via AP Photo) AP North Korea Military Parade In this image made from video released by KRT on Feb. 8, 2018, North Korean leader Kim Jong Un, center, watches the military parade in Pyongyang, Thursday, Feb. 8 2018. North Korea held a military parade and rally on Kim Il Sung Square on Thursday, just one day before South Korea holds the opening ceremony for the Pyeongchang Winter Olympics. (KRT via AP Photo) AP North Korea Military Parade This screen grab taken from North Korea's KCTV on February 8, 2018 shows North Korean leader Kim Jong Un speaking during a military parade in Kim Il Sung Square in Pyongyang. North Korea staged a military parade in Pyongyang on February 8 to mark the 70th anniversary of its armed forces, in a show of strength just a day before the Pyeongchang 2018 Winter Olympic Games open in the South. / AFP PHOTO / KCTV / KCTV / -----EDITORS NOTE --- RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / KCTV" - NO MARKETING - NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS - NO ARCHIVESKCTV/AFP/Getty Images AFP/Getty Images

Trump administration officials, like Mr Pompeo, have sought to make clear that the US is looking for North Korea to denuclearise. Mr Pompeo said at the briefing that Mr Kim had committed to such a promise. The secretary of state said Mr Trump will reject anything short of “complete, verifiable and irreversible denuclearisation of the Korean Peninsula.

“President Trump is hopeful. But he's also going into the summit with his eyes wide open,” Mr Pompeo said. He added that the US would work to guarantee North Korea's security should it denuclearize. Trump “is prepared to ensure a DPRK free of its weapons of mass destruction is also a secure North Korea,” Mr Pompeo said.

Mr Pompeo plans to stay in the region following the summit to meet with officials from Japan and South Korea and to travel to China, an important North Korean ally, to discuss the next steps involving Pyongyang.

Trump says he 'doesn't need to prepare' for North Korea summit

Mr Abe visited the White House on Thursday to coordinate strategy with Mr Trump ahead of next week’s summit, with Tokyo keen to make sure Japan's security concerns are not overlooked. Chief on his list of priorities for Mr Abe was the return of Japanese citizens abducted by – an issue he discussed with the US president at length last year.

While Mr Abe said he would prefer to discuss the issue with Mr Kim himself, he said he felt confident that Mr Trump would raise it with the North Korean leader in their upcoming meeting. The US president, he said, “is one of the leaders who understands the issue the most, the greatest”.

Mr Abe is also reportedly seeking an agreement with North Korea on their ballistic missile and chemical weapons programmes, on top of their nuclear weapons arsenal. Japanese citizens were forced to run for cover twice this summer, when the North sent ballistic missiles flying over the island of Hokkaido.

Mr Abe is open to arranging his own meeting with Mr Kim if the summit with Mr Trump goes well, having been largely absent from the recent burst of engagement between the US, South Korea and Pyongyang.

US officials had prepared for weeks for the 12 June nuclear summit, before Mr Trump abruptly called it off last month. Then-CIA Director Mr Pompeo met with North Korean leader Mr Kim in April, and the White House sent a team to Singapore to discuss details of the meeting as recently as last month.

Mr Trump announced that the meeting was back on again last week, after meeting with a senior North Korean envoy and receiving a letter from Mr Kim himself. He described the letter as “interesting,” but later said he had not opened it.

On Thursday, he revealed that the letter had been a simple greeting from Mr Kim.