US Defense Secretary Jim Mattis arriving for a news conference at Resolute Support headquarters in Kabul, Afghanistan. Thomson Reuters Defense Secretary Jim Mattis hinted that the United States still had military options left for dealing with North Korea, but did not elaborate when asked for details Monday.

Most experts think a military strike on North Korea would invite a devastating response from Pyongyang. The city of Seoul, South Korea, home to 25 million, is well within artillery range of the North, which would most likely use conventional artillery munitions and chemical weapons.

But according to Mattis, the Pentagon has a few tricks up its sleeve that wouldn't involve the decimation of Seoul.

When asked whether there was "any military option the US can take with North Korea that would not put Seoul at grave risk," Mattis responded, "Yes, there are, but I will not go into details."

Previously, Mattis said a war with North Korea would "involve the massive shelling of an ally's capital, which is one of the most densely packed cities on earth," referring to Seoul.

It's difficult to understand what the Pentagon could do to stop a North Korean nuclear program or take out its leader, Kim Jong Un, while preventing Pyongyang from fighting back. Artillery, rockets, missiles, and other munitions are scattered throughout the North — many in secret locations — and Kim's government maintains an ironclad hold on power.

And with every known military option — from launching Tomahawk cruise missiles to airstrikes — North Korea is likely to interpret any strike, however limited, "as a prelude to invading or overthrowing the government, even if the United States insists otherwise," The Atlantic reported, citing Daryl Press, a scholar of nuclear deterrence at Dartmouth College.

So what does Mattis have in mind? He wouldn't say, but he did let slip one interesting comment.

"Just to clarify, you said that there were possible military options that would not create a grave risk to Seoul," a reporter said later. "Are we talking kinetic options as well?"

"Yes, I don't want to go into that," Mattis said, agreeing that his closely held military option involved kinetic action, a euphemism to describe lethal military force.

President Donald Trump in a speech to the United Nations on Tuesday threatened to "totally destroy North Korea" if Pyongyang didn't back down on its nuclear provocations.

"It is an outrage that some nations would not only trade with such a regime, but arm, supply, and financially support a country that imperils the world with nuclear conflict," Trump said.