A new assessment from the Pentagon suggests a ground invasion would be necessary to locate and destroy all components of North Korea's nuclear weapons program.
The assessment is part of a letter signed by Rear Admiral and Vice Director of the Pentagon's Joint Staff Michael Dumont, addressed to Rep. Ted W. Lieu (D-Calif.) and Rep. Ruben Gallego (D-AZ), in response to their inquiry about the Pentagon's plan for military action against North Korea and the expected casualties should a conflict with Pyongyang actually materialize.
In a joint statement Lieu, along with 14 other house representatives who are veterans and mostly Democrats, said the notion of a ground invasion is "deeply disturbing and 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.
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."
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.