A ground invasion of North Korea is the only way to "'locate and destroy'" the reclusive country's nuclear weapons sites "with complete certainty," the Pentagon wrote in a letter to lawmakers.

The letter, outlining what a potential conflict with North Korea would look like, said that it is difficult to assess the "'best- or worst-case casualty scenarios'" from an attack from the North, noting the proximity of Seoul, South Korea's capital and largest city, to the Demilitarized Zone (DMZ).

Also obscuring casualty estimates is uncertainty about how much advanced warning the U.S. would have before a potential attack by Pyongyang, Rear Adm. Michael Dumont, the vice director of the Pentagon’s joint staff, wrote.

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The assessment put forth in the letter to Reps. Ted Lieu (D-Calif.) and Ruben Gallego Ruben GallegoHispanic caucus report takes stock of accomplishments with eye toward 2021 Senators call on Pentagon to reinstate funding for Stars and Stripes newspaper Hispanic Caucus campaign chief to mount leadership bid MORE (D-Ariz.) puts in stark terms what a conflict with North Korea could entail, warning that the country could turn to its stockpile of biological and chemical weapons in the event of war.

The Pentagon declined to detail how the U.S. would counter a nuclear strike by North Korea, but offered to give lawmakers a classified briefing on the matter.

The letter also reaffirms the Pentagon's support for the current regimen of sanctions and diplomatic pressure against North Korea, noting that there has not been a recent noticeable "change in the offensive posture of North Korea's forces."

"We remain postured to respond in the event of a provocation or conflict, and we have taken all appropriate measures to protect our allies, the Republic of Korea (ROK) and Japan, and U.S. Forces in those countries," the letter reads.

The assessment came in response to a letter sent to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE in September, in which Lieu and Gallego requested a casualty estimate of a potential conflict with North Korea.

"A decision to attack or invade another country will have ramifications for our troops and taxpayers, as well as the region, for decades," the lawmakers wrote at the time.

President Trump arrived in Japan, one of the U.S.'s closest allies in the region, on Saturday for the first stop on a 12-day trip across Asia, in which North Korea is certain to be a key issue of discussion. He is also set to visit South Korea, China, Vietnam and the Philippines.

The trip is Trump's first to Asia since taking office in January, and comes amid heightened tensions between the U.S. and North Korea, which has stepped up the pace of its weapons tests in recent months. Trump has also engaged in a bitter war of words with North Korean officials, and has repeatedly threatened military action against the country.