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The only way to locate and destroy with complete certainty all components of North Korea’s nuclear weapons program is through a ground invasion.

That blunt assessment from the Pentagon is in response to a letter from two Democratic congressmen asking about casualty assessments in a conflict with North Korea.

Rear Adm. Michael J. Dumont of the Joint Staff offered the assessment in response to a letter from Reps. Ted Lieu of California and Ruben Gallego of Arizona.

In a joint statement issued Saturday, 15 Democratic lawmakers and one Republican — all military veterans — called the assessment that a ground invasion would be required to destroy the North’s nuclear arsenal “deeply disturbing” and said that such an action “could result in hundreds of thousands, or even millions of deaths in just the first few days of fighting.”

The statement goes on to say that entering into a protracted and massive ground war with North Korea would be disastrous for U.S. troops and allies.

"There are no good military options for North Korea. Invading North Korea could result in a catastrophic loss of lives for U.S. troops and U.S. civilians in South Korea," the statement reads. "It could kill millions of South Koreans and put troops and civilians in Guam and Japan at risk."

The revelation comes as President Donald Trump starts his Asia trip, where the subject of North Korea's nuclear ambitions is expected to be high on the president's priority list.

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President Donald Trump and First Lady Melania Trump wave to U.S. service members at Yokota Air Base in Japan on Sunday KIMIMASA MAYAMA / EPA

The Pentagon letter to lawmakers also points out that North Korea may consider the use of biological weapons despite its international obligations, and is likely in possession of a chemical weapons stockpile.

It suggests a classified briefing would be required to discuss the details of how to eliminate North Korea's nuclear weapons located in deeply buried, underground facilities and counter North Korea's ability to retaliate with chemical and biological weapons.

A report by the Congressional Research Service released last month estimates as many as 25 million people on either side of the border, including more than 100,000 U.S. citizens, could be affected by an escalation of a military conflict on the Korean peninsula.

The members of Congress say it's their intent to have a full public accounting of the potential cost of war, so the American people have a solid understanding of what the commitment to military action entails.

"We must pursue every other option before even considering a massive ground invasion," their statement reads.

But the Pentagon says coming up with even the roughest casualty estimates can be challenging, adding that casualty estimates will vary significantly depending on the nature, intensity and duration of a North Korean attack, the readiness of alliance forces as well as how much advance warning there would be.

Dumont's letter points out the Joint Force fully supports economic and diplomatic pressure campaigns led by Secretary of State Rex Tillerson with regard to North Korea.

However, President Trump has previously told Tillerson that he was "wasting his time" trying to negotiate with North Korean leader Kim Jong Un, belittling previous negotiation efforts and adding that "only one thing will work."

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North Korea's leader Kim Jong Un KCNA / Reuters file

But aside from provocative statements against North Korea's leadership on his personal Twitter account and threats to unleash "fire and fury" on the rogue state, the administration has not explicitly talked about the possibility of a ground invasion before.

The lawmakers say if President Trump does intend to pursue a military option against North Korea, he must come to Congress as required by the Constitution.

"The stakes are too high and the potential outcome too grave for President Trump to violate his constitutional duty to come to Congress to authorize and oversee use of force," their statement reads.