WASHINGTON >>A ground invasion of North Korea is the only way to locate and destroy, with complete certainty, all components of leader Kim Jong Un’s nuclear weapons program, according to a Pentagon official.

“It is the most bleak assessment,” Sen. Dianne Feinstein, a member of the Senate Intelligence Committee, said Sunday.

Two House Democrats, in a letter to the Pentagon, had asked about casualty assessments in a possible conflict with North Korea, and Rear Adm. Michael J. Dumont of the Joint Staff responded on behalf of the Defense Department.

Dumont noted that the United States is evaluating North Korea’s ability to target heavily populated areas of South Korea with long-range artillery, rockets and ballistic missiles. He pointed out that Seoul, the South’s capital with a population of 25 million, is just 35 miles from the demilitarized zone that separates the two Koreas.

The amount of casualties would differ depending on the advance warning and the ability of U.S. and South Korea forces to counter these attacks, he told Reps. Ted Lieu, D-Calif., and Ruben Gallego, D-Ariz.

“A classified briefing would be the best place to discuss in detail the capability of the U.S. and its allies to discuss capabilities to counter North Korea’s ability to respond with a nuclear weapon and eliminate North Korea’s nuclear weapons located in deeply buried, underground facilities,” Dumont said. He also mentioned the possibility that chemical and biological weapons might be used by the North in case of a conflict.

Feinstein said she was pleased that Secretary of State Rex Tillerson was accompanying President Donald Trump during his Asia trip, when North Korea is a main issue.

“I think if he will stay the course and use diplomacy the way diplomacy can be used, then it might be possible to work something out. The worst alternative is a war which could become nuclear,” Feinstein told CNN’s “State of the Union.”

In a joint statement 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 that such an action “could result in hundreds of thousands, or even millions of deaths in just the first few days of fighting.”

“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 lawmakers said.

They also said the Trump administration “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. The President needs to stop making provocative statements that hinder diplomatic options and put American troops further at risk,” they said.