The summit between Donald Trump and Kim Jong-un should not lead to expectations of an immediate major breakthrough, the senior US official who last negotiated and almost achieved a deal with North Korea has cautioned.

There must, instead, be patience for talks that might last years, according to William Perry, as the landmark moment in recent international politics comes to pass.

It was, however, crucial that an agreement was achieved at the end of the process, as it would decide “whether there will be a military conflict over Korea,” said Mr Perry, who, as defence secretary in Bill Clinton’s administration, had persuaded Mr Kim’s father to shut down a nuclear complex and agree to curb Pyongyang’s missile programme.

The failure of the meeting between President Trump and the North Korean leader in Singapore would lead to accusations and recriminations and calls for military action, Mr Perry said.

Laying out a possible scenario, he said a conflict “could escalate into a nuclear catastrophe that could mean up to 10 million dead.

“North Korea has enough nuclear bombs, including thermonuclear bombs, to be able to destroy Seoul and South Korea, while they themselves will be destroyed. The outcome would be even worse if the conflict was to expand to China.”

Mr Perry carried out two rounds of negotiations with Kim Jong-il, the current leader’s father, the second time coming out of retirement at President Clinton’s request. On that occasion, in particular, things appeared to be moving in the right direction.

North and South Korean athletes appeared together in the same team at the Olympics, and Pyongyang sent their most senior military officer to Washington, recalled Mr Perry at a meeting in Geneva on the sidelines of forum on the prevention of nuclear proliferation.

“In the latter half of 1999, I spent four days in Pyongyang negotiating an agreement that would require it to give up its nuclear and long-range missile programme. North Korea was very positive about it,” he said.

“By the end of 2000 the deal was ready for signing by the heads of state. But a month later the Bush administration came to power and cut off all discussions with North Korea, thus walking away from the opportunity to stop North Korea’s nuclear programme.”

Conflict could escalate into a nuclear catastrophe that could mean up to 10 million dead, yes, 10 million William Perry, former negotiator under Bill Clinton

One of the men in the Bush administration who claims the most “credit” for the scuppering of the prospective deal was John Bolton, now Mr Trump’s new national security adviser.

Last month, after the summit between Trump and Kim Jong-un had been agreed upon, Mr Bolton demanded that North Korea follow the “Libyan model”, one under which Muammar Gaddafi voluntarily gave up his nuclear programme, only to be later deposed with the aid of US and Nato bombing and killed.

The North Koreans appeared to call off the meeting in reaction to Mr Bolton’s statement. It was reinstated following further talks.

Asked of his reaction to Mr Bolton’s intervention at the Luxembourg Forum Nuclear Conference, Mr Perry said: “We must remember that the president did not act according to Mr Bolton’s views and has gone for the meeting. But it is unfortunate that the president’s national security adviser takes this view.”

President Trump had threatened that he would walk away from the meeting if he believed that it would not work out, insisting he would know in the first minute on whether he could do a deal with Mr Kim.

“I’m hopeful that he’s right,” said Mr Perry. “But I could envisage a situation where both leaders leave in a fit of anger. So it really needs to end on a cordial note”.

Details of a deal would be too complex for the two leaders to nail down, the former defence secretary said.

“I don’t think it’s a bad idea to start off with big concepts first and then work it down to details.

I don’t think it’s a bad idea to start off with big concepts first and then work it down to details William Perry, former negotiator under Bill Clinton

“If it results in an agreement in getting a process started that leads to complete denuclearisation, that is a major achievement.”

Ideally, he added, meetings between officials tackling the technical aspects would continue, a process which “could take several years.”

Mr Perry recalled the North Koreans were, unsurprisingly, keen on economic assistance and normalisation of diplomatic relations when he negotiated with them, as well as being concerned over the prospect of a US attack.

“I offered them such an assurance in 1999 and they were very interested,” he said.

However, unlike then, Pyongyang now has a nuclear arsenal which it knows may be targeted by the US.

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So while talks to disarm continued, Mr Perry said, there could be a case for others who had taken part in the six-party talks – Japan, China, South Korea and Japan – to also give security assurances to North Korea.

Dr Viatcheslav Moshe Kantor, president of the Luxembourg Forum, said the near collapse of the deal on Iran’s nuclear programme hung over the Singapore talks.

Mr Trump recently pulled out of the Iran deal despite all the other international powers who were signatories to it – Britain, France, Germany, Russia and China – as well as the UN saying that it was working.

Dr Kantor said that “it would take Iran only a couple of years to build a nuclear warhead for its missiles if the deal were to go.

“It would be advisable for the US leadership to soften its policy on the Iran nuclear deal so as to protect the agreement from the threat of implosion.”

Reflecting on the Singapore summit, he said: “The progressive stiffening of sanctions by the UN Security Council and individual states proved most pivotal in bringing about a nuclear agreement with Iran and making Kim Jong-un amenable to a ‘sporting reconciliation’ and then to state-level meeting.