Donald Trump and Kim Jong Un's one-on-one meeting during their historic summit has been described as an "unacceptable danger to US national security" by retired American officials.

Both leaders were only accompanied by a pair of translators during the intimate huddle, which lasted for about 40 minutes at the very start of the summit.

Later in the morning, the meeting was expanded to include US president's chief of staff, national security adviser and secretary of state - as well as members of the North Korean leader's entourage.

Concerns have been raised over how the monumental talks, which began shortly after they met each other for the first time, took place without anyone on hand to take notes or witness what was being said.

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Paul Haenle, a former China director at the White House's national security council when Barack Obama and George W Bush were in power, said: "Bad idea. I could see Trump giving up a lot for very little in return."

Barry McCaffrey, a retired US army general, went further - warning America's security was at stake.

He said secretary of state Mike Pompeo and defence secretary Jim Mattis should have opposed the one-on-one meeting "in writing" - and called for the national security council to have a transcript of all interactions with the "brutal, nuclear armed dictator".

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Having aides present in meetings with adversaries is regarded as a way of providing a president with protection, as it prevents remarks from being misinterpreted.

But despite the apprehension over Mr Trump and Mr Kim's one-on-one talks, presidents have long met privately with foreign leaders - including enemies - with only interpreters on hand as witnesses.

Barack Obama occasionally held impromptu chats with fellow leaders at major global summits, with only their translators in attendance.

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Indeed, early on in his presidency, Mr Trump met Russian President Vladimir Putin during a conference in Germany, and only a translator from the Kremlin was present then.

Back in November 1985, Ronald Reagan met then-Soviet leader Mikhail Gorbachev in Switzerland, accompanied only by their trusted translators. That meeting in Geneva was only meant to last for 15 minutes, but it continued for a full hour.

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It is not known who has been translating for Mr Trump at the Sentosa Hotel in Singapore.

However, when Mike Pompeo met Mr Kim in Pyongyang earlier this year, translations were provided by Andrew Kim - a Korean-American who formerly served for the CIA in Seoul.

Andrew Kim has been spotted at the Singapore hotel where Mr Trump is staying. However, he was not listed as a member of the official US delegation.