Days after President Donald Trump referred to North Korean leader Kim Jong Un as “rocket man” and members of his administration made it clear military options against Pyongyang remained on the table, Defense Secretary James Mattis sought to calm growing fears of American intervention on the Korean Peninsula by arguing that the United States had military options at its disposal that wouldn’t necessarily spell disaster for allies in the region.

When pressed on Monday about escalating rhetoric from the White House regarding Kim’s nuclear ambitions, Mattis was asked during a press briefing at the Pentagon if there were any military strategies for dealing with North Korea that would protect Seoul, the South Korean metropolis home to 25 million people. In a terse comment that surprised international relations experts, Mattis hinted that there were, although he refrained from elaborating.

“Yes there are. But I will not go into details,” he said. When asked if they might include the use of lethal force, he replied: “I don’t want to go into that.”

If such military options do exist, they are largely new to analysts studying the Korean Peninsula.

“I don’t know what plan would not put Seoul at risk,” said Melissa Hanham, a senior research associate at the James Martin Center for Nonproliferation Studies. “The bottom line is: North Korea does have the artillery. It’s vague enough that I want to give [Mattis] the benefit of the doubt, but I cannot conceive of a way where you would militarily engage with North Korea and not put Seoul at risk.”

Seoul’s geographic proximity to North Korea has been a deterrent for U.S. military intervention on the Korean peninsula. Pyongyang has thousands of traditional arms ― rocket launchers and cannons ― buried in the mountains just north of the Demilitarized Zone that could rain down upon Seoul’s skyscrapers if Kim felt threatened. The weaponry, built up in the decades since the end of the Korean War, is heavily fortified and would be almost impossible to take out in one fell swoop.

Jonathan Pollack, a senior fellow at the Brookings Institution specializing in Korea and China, was puzzled by Mattis’ statements, arguing that the defense secretary is usually a “voice of real reason” in the Trump administration.

“He’s a very sober, careful guy,” Pollack said. “Frankly, I haven’t got a clue about what he’s talking about ... He knows what the terrain looks like, he knows what the risks are, he knows how deeply buried and dispersed the North Koreans are... I guess I’m having difficulty connecting the dots.”

Mattis’ comments could unnerve American allies in South Korea as they once again raise the prospect of unilateral action by the U.S. against the north, Pollack added.

South Korean President Moon Jae-in has openly warned the U.S. against doing just that, and has worked to use diplomacy and deterrence to counter Kim. The United Nations recently imposed harsh new sanctions against Pyongyang and the U.S. and South Korea staged joint bombing drills over the Korean Peninsula this week in a show of force.

But Trump officials continue to raise the specter of military action and signal that they are losing patience with Kim. North Korea recently conducted its second test of a ballistic missile that flew over Japan, following up on its sixth, and by far the most powerful, nuclear test earlier this month.

“If our diplomatic efforts fail, though, our military option will be the only one left,” Secretary of State Rex Tillerson said on CBS’s “Face the Nation” on Sunday. “To be clear, we seek a peaceful solution to this.”

Nikki Haley, the U.S. Ambassador to the U.N., echoed those sentiments on CNN that same day, saying: “If North Korea keeps on with this reckless behavior, if the United States has to defend itself or defend its allies in any way, North Korea will be destroyed.”

However, it’s unlikely such destruction would be one-sided.

Vipin Narang, an associate professor at MIT studying nuclear proliferation, said any expectation otherwise seemed like “extremely heroic assumptions.”

“It is more likely that we cannot have 100 percent certainty in disarming the country,” he said in an email. “Any attack on North Korea that doesn’t fully annihilate its conventional and WMD forces exposes Seoul ―and U.S. forces and dependents, Japan, U.S. territories, and even possibly the U.S. homeland ― to potentially massive destruction.”