Speaking after a meeting with senior South Korean defense officials on Saturday, Defense Secretary Jim Mattis offered unequivocal support for American allies in the face of threats from North Korea.

"Make no mistake," Mattis told reporters, "any attack on the United States or our allies will be defeated, and any use of nuclear weapons by the North will be met with a massive military response that is effective and overwhelming."

But despite the reassurances offered Saturday, the defense secretary's visit also underlined arguably the biggest problem in containing the threat from North Korea: the conventional weaponry at the border that would put millions of South Koreans at risk if any conflict were to break out.

Mattis got a sense of this firsthand the day before, when he visited the demilitarized zone (DMZ) that separates the two countries with South Korean Defense Minister Song Young-Moo. While there, Song pointed to North Korea's long-range artillery in the distance and suggested it would be "unfeasible" to defend against them in a conflict.

"Understood," Mattis responded, according to Reuters.

Over the past seven decades, North Korea has amassed a huge number of artillery pieces along its border with South Korea. Joseph S. Bermudez Jr., a senior imagery analyst at North Korea-focused website 38 North, estimated earlier this year that the Second Corps of the Korean People's Army stationed at Kaesong on the northern side of the DMZ has about 500 artillery pieces alone. Other estimates of the total number of weapons of this sort puts it at 8,000.



U.S. Defense Secretary Jim Mattis, center front, and South Korean Defense Minister Song Young-moo, left, visit Observation Post Ouellette in Paju, near the truce village of Panmunjom in the demilitarized zone on the border between North and South Korea, on Friday. (Jung Yeon-Je/AP)

Though Pyongyang's nuclear weapons program has captured more international attention recently, this artillery shapes much of the discussion surrounding any possible conflict with North Korea.

Seoul lies just 30 miles from the DMZ, comfortably within range for the artillery at Kaesong. The South Korean capital is one of the largest cities in the world, with a population of 25 million. Experts suggest that in the event of war, North Korea could use these conventional weapons to hammer the city, potentially leading to enormous death tolls.

A 2012 study by Roger Cavazos of the Nautilus Institute estimated there could be more than 2,800 fatalities in the initial volley. In total, Cavazos wrote, 64,000 people could be killed in the first day of any conflict. Another estimate recently given to U.S. lawmakers suggested that as many as 300,000 could die in the early days of a conflict, even if nuclear weapons are not used.

Many analysts have suggested it would be difficult and time-consuming to destroy these weapons if war broke out. "When it comes to the artillery, there's nothing we can do against the artillery directly to prevent bombardment of Seoul," said Van Jackson, an expert on North Korean security issues at Victoria University in New Zealand.

Despite these concerns, the Trump administration has downplayed the threat posed to the South Korean capital. In September, when asked if there were any military options on the table that would not put Seoul at grave risk, Mattis said that there were, "but I will not go into details."

Asked about the possibility of a preventive strike on North Korea, Mattis told reporters on Saturday there were many different military options that "realistically reduce that threat [to South Korea] as low as possible."

"And yes, we do have those options," he added.

Robert E. Kelly, an expert on North Korea at Pusan National University in South Korea, said it was not clear what options Mattis was referring to. "The military option is traditionally considered really risky," Kelly wrote in an email.

Jackson said there are "a couple of military options [against North Korea] that wouldn't necessitate retaliation against Seoul," such as a limited strike or covert operation.

"It has to be an isolated attack, not part of some multistage strike plan or military campaign," Jackson added. "It has to have an accompanying media blackout — we can't take ownership of it."

Mattis is in South Korea as part of a week-long Asia trip. The visit, his second since taking office in January, comes after a number of provocative moves by the North Korean military, including a number of missile launches and a nuclear test in early September. Last week, a North Korean official reiterated a threat to stage an atmospheric nuclear test over the Pacific Ocean, telling CNN the warnings should be taken "literally."

"North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs," Mattis told reporters Saturday, adding that he could not imagine a "condition under which the United States would accept North Korea as a nuclear power."

However, Mattis reiterated that the military options being considered were "designed to buttress diplomats' efforts to maintain a deterrent stance and denuclearize the peninsula."

Read more:

Today’s coverage from Post correspondents around the world

Like Washington Post World on Facebook and stay updated on foreign news