Rear Admiral Michael Dumont expressed the opinion on behalf of the Joint Chiefs of Staff in a letter to Congressman Ted Lieu.
Mr Dumont said calculating “even the roughest” potential casualty figures would be extremely difficult.
He also gave some detail on what the first hours of a war would involve.
“The only way to ‘locate and destroy – with complete certainty – all components of North Korea’s nuclear weapons programs’ is through a ground invasion,” he wrote in response to Congressman Lieu’s questions about a potential conflict.
The risks involved included a potential nuclear counter-attack by North Korea while US forces attempted to disable its “deeply buried, underground facilities”, he said.
“A classified briefing is the best venue for a detailed discussion,” he added.
The Joint Chiefs of Staff directly advise the president of the United States on military matters.
In a statement with more than a dozen other military veterans turned congressmen, Mr Lieu, a Democrat, said the assessment was “deeply disturbing” and warned that a conflict “could result in hundreds of thousands, or even millions of deaths in just the first few days of fighting.”
“Their assessment underscores what we’ve known all along: there are no good military options for North Korea,” the statement said.
The letter was published as Donald Trump begins his mammoth tour of Asia, during which the North Korean threat is expected to be a major topic of discussion. (Courtesy BBC)