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,’’ Senator 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 Admiral Michael J. Dumont of the Joint Staff responded for the Defense Department.

Dumont said 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 number of casualties would differ depending on the advance warning and the ability of US and South Korea forces to counter these attacks, he told Representatives Ted Lieu, Democrat of California, and Ruben Gallego, Democrat of Arizona.

‘‘A classified briefing would be the best place to discuss in detail the capability of the US and its allies 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.

Dumont 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 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.