(CNN) Despite new threats from the Trump administration, the US still has no real, practical military option on North Korea, analysts say.

After Pyongyang tested a nuclear weapon Sunday, US Defense Secretary James Mattis said the US has "many military options" on North Korea and threatened Pyongyang with "a massive military response" if a US territory or US allies are targeted.

But any US military action puts millions of civilians in the South Korean capital of Seoul at risk, analysts say, and is therefore very unlikely to happen.

"We always have military options, but they're very ugly," said Mark Hertling, a retired US Army general and CNN military analyst.

Weapons pointed at Seoul

For years, the problem with any US military action against North Korea's nuclear program has been the mass of conventional artillery Pyongyang keeps in range of Seoul, a metropolitan area of 25 million people.

Experts say North Korea could kill tens of thousands of those civilians, if not exponentially more, using that artillery in retaliation for a US strike on the North.

Pyongyang's test Sunday of what is thought to be its most powerful nuclear weapon doesn't change that fact, experts say. But some also say the test doesn't mean North Korea is any more of an immediate threat to the US or its allies.

Hertling said that means the US doesn't need to do anything drastic to counter the current North Korean threat.

"Does North Korea threaten the existence of the United States or any of our allies right now? No, they don't," he said.

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Technological challenges

While North Korea did explode a nuclear weapon underground, and it has this summer tested what experts believe are ballistic missiles capable of reaching the US mainland, it hasn't shown it can put the two together in a system that works.

"A successful test means they can build a weapon, not that they have any ready for immediate use," said Carl Schuster, a former director of operations at the US Pacific Command's Joint Intelligence Center.

But others have cautioned that the Kim Jong Un regime must be taken at its word because no one can be certain they have a nuclear-tipped missile that works until they actually try to hit something with one.

"If you attack them after they have the nuclear weapons, it's not a preventive war. It's just a plain old nuclear war," Jeffrey Lewis, a nonproliferation expert at the Middlebury Institute of International Studies' James Martin Center for Nonproliferation Studies, said on a recent podcast

Bruce Bennett, a senior researcher at the RAND Corporation who specializes in North Korea, said letting Pyongyang develop a working nuclear-tipped missile leaves the US open to a kind of nuclear blackmail.

"North Korea might tell the United States to not send conventional forces to South Korea, threatening to detonate nuclear weapons on US cities if the US does send those forces," Bennett said.

What would be needed for strike

Even if that's a possibility, analysts say the US is in no position right now to wage the kind of military campaign that could bring battlefield success on the Korean Peninsula. It would need weeks, if not months, to get needed additional troops and equipment to the region.

Those would include aircraft such as B-2 and B-52 bombers that would operate out of the US Air Force base on Guam , and F-22 stealth fighters would be part of any first strike, experts say.

Additional warships and submarines equipped with Tomahawk missiles would be needed to suppress North Korean air defenses so the bombers could operate, they say.

And then ground troops would be needed in far greater numbers that are now available to take control of sites inside North Korea where nuclear weapons might survive a first strike.

"No additional forces are present," Schuster said Monday.

But the US does have the forces such as the THAAD and Aegis missile defense systems in place to protect South Korea and Japan from a North Korean first strike, Hertling said.

And that means there's still time to get diplomacy to work, he said.

"We're still controlling the clock here. It's within our ability to keep the initiative," he said.