Central Intelligence Agency Director Mike Pompeo and a team at the CIA have been working through intelligence back-channels to make preparations for the summit, the officials said. American and North Korean intelligence officials have spoken several times and have even met in a third country, with a focus on nailing down a location for the talks. Although the North Korean regime has not publicly declared its invitation by Kim Jong Un to meet with Trump, which was conveyed last month by a South Korean envoy, several officials say North Korea has since acknowledged Trump's acceptance, and Pyongyang has reaffirmed Kim is willing to discuss the denuclearization of the Korean peninsula.The North Koreans are pushing to have the meeting in their capital, Pyongyang, the sources said, although it is unclear whether the White House would be willing to hold the talks there. The Mongolian capital Ulaanbaatar has also been raised as a possible location, the sources said. The talks between intelligence officials are laying the groundwork for a meeting between Pompeo and his North Korea counterpart, the head of the Reconnaissance General Bureau, in advance of the leaders' summit. Once a location is agreed upon the officials said that the date will be set and the agenda discussed in greater detail. Officials said the decision to use the already existing intelligence channel was more a facet of Pompeo's current status as CIA director as he awaits confirmation as secretary of state than a reflection of the content of the discussions. Pompeo is expected to begin the process of Senate confirmation in the next several weeks.One of Trump's most trusted national security advisers, Pompeo has led efforts to prepare for the summit, which Trump has pressed his aides to organize. If he confirmed, he will assume oversight of the diplomatic preparations. As recently as this weekend, Trump told associates he was looking forward to the summit, which he agreed to on the spot when presented the invitation from Kim. The timeline, however, remains unknown. Officials said the current target is late May or even June.Trump is due to meet in two weeks with Japanese Prime Minister Shinzo Abe at his Mar-a-Lago estate. Abe is expected to come bearing a list of concerns about opening talks with Kim. The New York Times first reported last month that the CIA was taking the lead in preparing for the Trump-Kim summit.Officials said the participation of the North Koreans in the preparatory talks give them more confidence that Kim is serious about meeting. Until the talks between US and North Korean intelligence officials began in earnest, Trump and his aides have relied partly on the characterizations of the South Koreans, which have experienced a rapprochement since the Olympic games held in Pyongchang in February that led to Kim's historic invite to Trump. The Chinese have also provided a briefing to the White House after Kim and President Xi Jinping met in Beijing late last month. State Department officials continue to communicate with the North Koreans though their mission to the United Nations, discussions which are referred to as the "New York channel." The talks with North Korea are informing coordination among government agencies which are preparing for the summit, an effort led by Matthew Pottinger, the top Asia official at the National Security Council. Incoming national security adviser John Bolton, who starts work at the White House on Monday, is expected to assume a large role in the planning for the talks, along with Pompeo.At the State Department, leading the diplomatic effort are acting Assistant Secretary Susan Thornton and deputy special representative for North Korean policy Mark Lambert, who speaks with North Korean officials through the "New York channel." Their work includes scouting potential locations, coming up with names of US officials who can help staff the talks and pouring over records on previous negotiations with North Korea. They are also leading diplomacy with South Korea, as well as Japan, China and Russia.