Singapore, by contrast, is neutral ground, nearly 3,000 miles from Pyongyang, and not a treaty ally of the United States, like South Korea, Japan or the Philippines.
Both countries have embassies there, United States Navy warships call at Singapore’s port and North Korea has operated trading companies there, though they have been shut down because of sanctions against Pyongyang.
“North Korea will have a comfort level there that they just don’t have in other countries,” said Franklin L. Lavin, who served as the American ambassador to Singapore under President George W. Bush.
A large C.I.A. station is also in Singapore, another former official said, and American spies meet regularly with their North Korean counterparts as part of an intelligence channel between the two countries. Negotiations for the meeting between Mr. Trump and Mr. Kim have largely been carried out through this intelligence channel.
Singapore — unlike, say, Mongolia — is also viewed as safe for both leaders. “I’m sure North Korea would have preferred the DMZ,” said Joseph Y. Yun, who recently retired as the State Department’s senior diplomat on Korea. “But it’s an orderly place. No feisty protesters.”
For Mr. Kim, there is one awkward element: Singapore borders Malaysia, where his half brother, Kim Jong-nam, was killed after two women attacked him with a deadly nerve agent at Kuala Lumpur International Airport. The North Korean government is widely suspected of ordering the attack.
As for the questions about Mr. Kim’s ability to fly, he just met with President Xi Jinping of China in the Chinese coastal city of Dalian — the first time a North Korean leader had flown abroad in 32 years. That eased doubts about whether he can get to Singapore, though there are still questions about the range of North Korea’s Soviet-era aircraft.