And the number could be even higher if the hermit state used nuclear weapons during the clash, said Robert Givens, a former US Air Force Brigadier General who spent four years stationed on the Korean Peninsula.
Mr Givens said the staggering figure came from a Pentagon estimate.
He added: “Too many Americans have the view that it would be like the invasion of Iraq or Afghanistan, or like combat operations in Libya or Syria, but it wouldn’t remotely resemble that.
“There is only one way that this war ends, with North Korea’s defeat — but at what cost?”
Tensions have been escalating between the US and Pyongyang over the past few months as North Korea has carried out tests on an intercontinental ballistic missile and six other nuclear bombs.
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But it is not known if they have the technology to combine the two and launch a nuclear weapon.
The US says satellite imagery has also detected a number of North Korean military aircraft moving to the North’s east coast.
President Trump has done nothing to de-escalate the war of words between the two leaders, and on Tuesday he said the US would be prepared to use “devastating” military action if necessary.
On Monday, the North Korean Foreign Minister Ri Yong Ho claimed Donald Trump had “declared a war” on his country, and threatened to shoot down US aircraft flying off its coastline even if they were in international airspace.
Last week the US President sent a warning shot to “madman” Kim Jong-un by threatening North Korea will “be tested like never before”.
The North Korean despot lashed out at Mr Trump, calling him ”mentally deranged” before claiming President’s comments were ”the most ferocious declaration of a war in history”.
He added: ”Whatever Trump might have expected, he will face results beyond his expectation. I will surely and definitely tame the mentally deranged US dotard with fire.”
Mr Trump asked Theresa May to increase pressure on the rogue regime when they met at the United Nations, in New York.
In a speech, the Prime Minister later said the international community “must be prepared to take all necessary measures” to “force Kim Jong-Un to change his ways”.
Colonel Robert Manning, US Pentagon spokesman, recently said: “If North Korea does not stop their provocative actions, we will make sure that we provide options to the President to deal with North Korea.”
Nikki Haley, US Ambassador to the UN recently claimed Kim Jong-un is “begging for war”.
The North Korean army appears to be a formidable force with 1.2 million soldiers and 100,000 special forces trained to infiltrate South Korea.
And although military experts say the country’s equipment dates back to the Soviet era, Pyongyang may have more tanks than the US and more artillery pieces than China.
Mr Givens added: “In an all-out conquest for regime survival, they will come after the United States.
“They are not going to win, but they will try – I guarantee that.”