The only way to locate and secure all of North Korea’s nuclear weapons sites “with complete certainty” is through an invasion of ground forces, and in the event of conflict, Pyongyang could use biological and chemical weapons, the Pentagon told politicians in a new, blunt assessment of what war on the Korean Peninsula might look like.

The Pentagon, in a letter to politicians, said that a full discussion of US capabilities to “counter North Korea’s ability to respond with a nuclear weapon and to eliminate North Korea’s nuclear weapons located in deeply buried, underground facilities” is best suited for a classified briefing.

The letter also said that Pentagon leaders “assess that North Korea may consider the use of biological weapons” and that the country “has a long-standing chemical weapons program with the capability to produce nerve, blister, blood and choking agents.”

The Pentagon repeated that a detailed discussion of how the United States would respond to the threat could not be discussed in public.

The letter was written by Rear Admiral Michael J Dumont, the vice director of the Pentagon’s Joint Staff, in response to a request for information from two House members about “expected casualty assessments in a conflict with North Korea,” including for civilians and US and allied forces in South Korea, Japan and Guam.

“A decision to attack or invade another country will have ramifications for our troops and taxpayers, as well as the region, for decades,” Ted Lieu (Democrat, California) and Ruben Gallego (Democrat, Arizona) wrote to the Pentagon. “We have not heard detailed analysis of expected US or allied force casualties, expected civilian casualties, what plans exist for the aftermath of a strike — including continuity of the South Korean Government.”

The Pentagon said that calculating “best-or worst-case casualty scenarios” was challenging and would depend on the “nature, intensity and duration” of a North Korean attack; how much warning civilians would have to get to the thousands of shelters in South Korea; and the ability of US and South Korean forces to respond to North Korean artillery, rockets and ballistic missiles with their own retaliatory barrage and air strikes.

The letter noted that Seoul, the South Korean capital, is a densely populated area with 25 million residents.

Any operation to pursue North Korean nuclear weapons would likely be spearheaded by US Special Operations troops. Last year, President Barack Obama and then-Defence Secretary Ashton B Carter gave US Special Operations Command a new, leading role coordinating the Pentagon’s effort to counter weapons of mass destruction. SOCOM did not receive any new legal authorities for the mission but gained influence in how the military responds to such threats.

Elite US forces have long trained to respond in the case of a so-called “loose nuke” in the hands of terrorists. But senior officials said SOCOM is increasingly focused on North Korea.

Dumont said the military backs the current US strategy on North Korea, which is led by Secretary of State Rex Tillerson and focuses on ratcheting up economic and diplomatic pressure as the primary effort to get North Korean leader Kim Jong Un to stop developing nuclear weapons. Tillerson, Defence Secretary Jim Mattis and the Joint Chiefs of Staff chairman, General Joseph F Dunford Jr, have emphasized that during trips to Seoul this year.

In contrast, Donald Trump, who goes unmentioned in the Pentagon letter, has taunted Kim as “Rocket Man” and expressed frustration with diplomatic efforts, hinting that he is considering preemptive military force.

“I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man,” Trump tweeted on 1 October, adding, “Save your energy Rex, we’ll do what has to be done!”

On 7 October, Trump added in additional tweets that North Korea had “made fools” of US negotiators. “Sorry, but only one thing will work!” he said.

Mattis and other Pentagon leaders have often cited the grave threat faced by Seoul, but the military much less frequently draws attention to its plans for an underground hunt for nuclear weapons.

Air Force Colonel Patrick Ryder, a Pentagon spokesman, said that Dumont and other Pentagon officials had no additional comment about the letter.

A senior US military official in South Korea, speaking on the condition of anonymity to discuss ongoing operations, said that while the 28,500 US troops in South Korea maintain a high degree of readiness, he “has to believe” that North Korea does not want a war, given all of the nations aligned against it.

“If you open the history books, this is not the first time that we’ve been in a heavy provocation cycle,” the official said. On the side of South Korea and the United States, he said, “there is no action taken without extreme consideration of not putting this in a position where a fight is going to happen.”

Dumont’s letter also notes that “we have not seen any change in the offensive posture of North Korea’s forces.”

A statement by 16 politicians, released simultaneously with the Pentagon letter, urged Trump to stop making “provocative statements” that impede diplomatic efforts and risk the lives of US troops.

The Pentagon’s “assessment underscores what we’ve known all along: There are no good military options for North Korea,” said the statement, organized by Lieu and Gallego and signed by 14 other members of Congress who are veterans, all but one of them Democrats.

In a telephone interview, Lieu said that the intent of asking the Pentagon for information was to spell out the cataclysmic consequences of war with North Korea and the aftermath.

“It’s important for people to understand what a war with a nuclear power would look like,” said Lieu, citing estimates of 300,000 dead in the first few days alone. More than 100,000 Americans are potentially at risk.

Lieu, who spent part of his time in the Air Force on Guam preparing for military action against North Korea, called the letter a confirmation that a conflict would result in a “bloody, protracted ground war.” The Joint Chiefs, he believes, are “trying to send a message to the American public,” he said.

“This is grim,” Lieu said. “We need to understand what war means. And it hasn’t been articulated very well. I think they’re trying to articulate some of that.”

Gallego said that he wanted information because of what he sees as a cavalier attitude in the White House about military action in North Korea. The idea that a ground invasion would be needed to secure nuclear weapons is eye-opening, he said, and raises the possibility of the US military losing thousands of troops.

“I think that you’re dealing with career professionals at the Pentagon who realize that the drumbeats of war could actually end up leading us to war,” he said. “They want to make sure that there is full transparency and information out there about what can occur if our civilian leaders make wrong calculations.”

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The Pentagon letter also notes the possibility of “opposition from China or Russia.”

“The Department of Defence maintains a set of up-to-date contingency plans to secure our vital national security interests,” Dumont wrote. “These plans account for a wide range of possibilities, including third-party intervention, and address how best to ‘contain escalation.’ ”

The letter says that both “Russia or China may prefer to avoid conflict with the United States, or possibly cooperate with us.”