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People watch a TV screen showing a local news program reporting about North Korea's missile launch at Seoul Train Station in Seoul, South Korea, on Aug. 30, 2017.Lee Jin-man/The Associated Press

The international community pledged to strike back with an iron fist after North Korea's sixth nuclear test, with Japan and South Korea agreeing to "an entirely different level" of retaliatory measures including harsh new sanctions.

But amid signs that North Korea is preparing another long-term missile test, it was South Korea's quick decision Monday to allow the installation of four more U.S. missile defence launchers systems that best underscored the new reality that has rocked the tension-riven region: Pyongyang is now widely believed to possess a sufficiently credible long-range nuclear weapon that the list of options is being whittled down to strengthening defences, and hoping Pyongyang doesn't next start building tactical battlefield nukes.

"The majority view of the technical community is the North Koreans already have the capability to launch a missile to reach at least the West Coast of the homeland of the United States," said Tong Zhao, a fellow in the Nuclear Policy Program at the Carnegie–Tsinghua Center for Global Policy.

Explainer: What we know so far about North Korea's missile program

"It's already capable of striking nuclear warheads towards enemies. It's just the reality that everyone has to face."

In addition to bolstering the U.S. Terminal High Altitude Area Defence anti-missile installation on its soil, South Korea on Monday said it will press the U.S. to deploy an aircraft carrier group, bombers and other "strategic assets" to the region, the country's defence ministry said. Seoul also staged a live-fire drill Monday meant to simulate an attack on North Korean nuclear facilities, after U.S. Defence Secretary Jim Mattis said Washington would order a "massive military response" if Pyongyang threatens its territory or that of its allies.

But the spectre of military reprisal came as technical experts and political analysts said North Korea should be seen as credible in claiming that it has tested a hydrogen bomb, one it said was small enough to be fitted into a long-range missile. South Korean experts said the Sunday detonation was more than three times more powerful than the bomb dropped on Hiroshima.

What debate remains primarily concerns the country's ability to accurately target such a weapon – a question that has grown less important given that a hydrogen bomb does not need to be precise to inflict serious damage – and whether the country's warhead is light enough that it can also reach the U.S. east coast.

"But there is really no big difference in terms of the strategic implications," said Mr. Zhao.

It's now clear that for the U.S., any attack on North Korea risks "losing a city or two" in retribution on its own soil, said Shea Cotton, a research associate with the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies.

"So any sort of military option is off the table."

The successful detonation of a hydrogen bomb would also do grave damage to expectations that negotiations might one day convince Pyongyang to lay down its nuclear weapons – even if the country has not yet fully demonstrated an intercontinental ballistic missile with complete re-entry and guidance abilities.

"So they may still have some hurdles, but the number dwindles," sad Scott LaFoy, an independent imagery analyst who studies ballistic missile technology. And, he said, "they've shown off enough that we can reasonably assume that they are either at that point (sans operational test) or very close."

At most, North Korea might need a "maximum time horizon" of six months to a year to finalize its long-range nuclear missile development – a short enough span that any international opposition is likely to be futile, said Go Myong-Hyun, a research fellow at the Asan Institute for Policy Studies.

That "means we are already there," in terms of the country's long-range nuclear capability.

North Korea should not, however, be seen as having closed any opening for talks, he said. Pyongyang has not made good on its threat to fire missiles into waters off Guam, and the nuclear test "doesn't threaten the United States directly," Mr. Go said.

After the test, too, North Korean supreme leader Kim Jong-un called on his scientists to "conduct the campaign for successfully concluding the final-stage research and development" in its nuclear program.

Those words appear to be carefully chosen. Mr. Kim could have declared his country a full-fledged nuclear weapons power.

But in saying that work remains, "North Korea is leaving a little bit of space, to send a message to the U.S. and China that there's room for negotiation: 'You can stop us before we get there'," Mr. Go said.

That "actually increases the chance of having talks with the international community, in my view."

Still, Seoul on Monday warned that North Korea appears to be preparing for another missile test, an indication of the degree to which the country continues to see ways it can expand its weapons capabilities.

And at a UN Security Council emergency session Monday, U.S. ambassador Nikki R. Haley said North Korea's leader "is begging for war."

"North Korea will likely to continue testing until they prove full weaponization and operation-ality of their nuclear assets," said Ken Jimbo, a security expert at Japan's Keio University.

What Pyongyang possesses today is a basic nuclear deterrence. If Mr. Kim desires, he can direct his ambitions toward far more sophisticated weapons: road-mobile inter-continental ballistic missiles powered by solid fuel that can be fired with little notice; small tactical weapons that can be used on the battlefield; or improved submarine-based missiles that are difficult to detect or destroy.

"So North Korea can do much worse by greatly enlarging and diversifying its nuclear arsenal," said Mr. Zhao.

The international community should now direct its attention to preventing that possibility, even if it can't keep Mr. Kim from having the nuclear weapons he has already built, he said.

"To deter North Korea, to contain its further development – that's the only realistic option."

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