A top Pentagon official has said the only sure way of eliminating North Korea’s nuclear weapons capabilities would be by putting U.S. boots on the ground — a move that some worry could prompt Pyongyang to use biological, chemical and even nuclear weapons against Japan and South Korea.

“The only way to ‘locate and destroy — with complete certainty — all components of North Korea’s nuclear weapons programs’ is through a ground invasion,” Rear Adm. Michael J. Dumont, vice director of the Joint Chiefs of Staff wrote in a blunt assessment to U.S. lawmakers that was released Saturday on the realities of reining in Pyongyang’s nuclear ambitions.

Dumont’s letter came in response to questions by U.S. Reps. Ted Lieu of California and Ruben Gallego of Arizona in regards to military planning and casualty estimates in the event of conflict with the nuclear-armed North.

Dumont said that a detailed discussion of U.S. 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,” would be best suited for a classified briefing.

The military, Dumont wrote, “would be happy to join the Intelligence Community to address these issues in a classified briefing.”

His letter also noted that the North “may consider the use of biological weapons as an option, contrary to its obligations under the Biological and Toxic Weapons Convention,” adding that it continues to bolster its research and development capabilities in this area.

North Korea, the letter went on, “has a long-standing chemical weapons program with the capability to produce nerve, blister, blood and choking agents and it likely possesses a CW stockpile.”

The country “probably could employ CW agents by modifying a variety of conventional munitions, including artillery and ballistic missiles, though whether it would so employ CW agents remains an open question,” Dumont said, again noting that a detailed discussion would need to be held in a classified setting.

The Pentagon also said it was “challenging” to calculate “best- or worst-case casualty estimates” for any conventional or nuclear attack, citing the nature, intensity and duration of any strike, as well as how much advance warning is given.

In a joint statement in response to the letter, 16 U.S. lawmakers — all veterans — called the prospect of a ground invasion “deeply disturbing.”

“The Joint Chiefs of Staff has now confirmed that the only way to destroy North Korea’s nuclear arsenal is through a ground invasion,” they wrote. “That is deeply disturbing and could result in hundreds of thousands, or even millions of deaths in just the first few days of fighting.”

These estimates echoed a report by the Congressional Research Service released late last month that said renewed conflict on the Korean Peninsula could kill hundreds of thousands of people in the first few days alone, a figure that excluded the potential use of nuclear weapons.

Even if North Korea “uses only its conventional munitions, estimates range from between 30,000 and 300,000 dead in the first days of fighting,” the report said, citing North Korea’s ability to fire 10,000 rounds per minute at Seoul.

More pressingly for Japan, the report noted is that “Pyongyang could also escalate to attacking Japan with ballistic missiles, including the greater Tokyo area and its roughly 38 million residents.

“The regime might see such an attack as justified by its historic hostility toward Japan based on Japan’s annexation of the Korean Peninsula from 1910 to 1945, or it could launch missiles in an attempt to knock out U.S. military assets stationed on the archipelago,” the report said. “A further planning consideration is that North Korea might also strike U.S. bases in Japan (or South Korea) first, possibly with nuclear weapons, to deter military action by U.S./ROK forces.”

ROK is the formal name for South Korea.

U.S. leader Donald Trump, who kicked off his first trip to Asia as president with a visit to Japan on Sunday, has regularly noted that all options, including military action, remain on the table.

The global community has been ramping up pressure on North Korea after it conducted its sixth and most powerful nuclear test so far on Sept. 3. In September, the U.N. Security Council strengthened its sanctions, including export bans as well as asset freezes and travel bans on various officials.

For his part, Trump, together with Prime Minister Shinzo Abe, has taken an approach of “maximum pressure” in dealing with Pyongyang.

But Trump, known to derisively refer to North Korean leader Kim Jong Un as “rocket man,” has also variously threatened North Korea with “fire and fury” and to “totally destroy” the country of 25 million people if the United States is forced to defend itself or its allies, including Japan.

This possibility of military action has stoked alarm among allied nations and within the U.S. Congress, including questions about planning and the aftermath of such a move.

“It is our intent to have a full public accounting of the potential cost of war, so the American people understand the commitment we would be making as a nation if we were to pursue military action,” the 16 lawmakers wrote in their statement.

The Trump administration, the lawmakers said, “has failed to articulate any plans to prevent the military conflict from expanding beyond the Korean Peninsula and to manage what happens after the conflict is over.”

“With that in mind, the thought of sending troops into harm’s way and expending resources on another potentially unwinnable war is chilling,” they said. “The President needs to stop making provocative statements that hinder diplomatic options and put American troops further at risk.”

The United States has roughly 50,000 troops stationed in Japan and 28,500 based in South Korea.

“Invading North Korea could result in a catastrophic loss of lives for U.S. troops and U.S. civilians in South Korea,” the lawmakers said. “It could kill millions of South Koreans and put troops and civilians in Guam and Japan at risk.

“As Veterans, we have defended this nation in war and we remain committed to this country’s security. We also understand that entering into a protracted and massive ground war with North Korea would be disastrous for U.S. troops and our allies,” they said. “The Joint Chiefs of Staff, it appears, agree. Their assessment underscores what we’ve known all along: There are no good military options for North Korea.”