SEOUL (AFP) - North Korea's nuclear threat has grown significantly following its latest and largest nuclear test and a series of missile launches, analysts say, with some South Korean newspapers even theorising about an atomic attack on Seoul.

The South Korean capital stayed calm Saturday (Sept 10), with residents immune to near-daily threats from their neighbour, but newspapers and analysts saw Friday's test as a game-changer.

With a force of 10 kilotons, the blast was two-thirds the size of the atomic bomb that destroyed Hiroshima in August 1945. It took place just eight months after the previous detonation.

More importantly, the North claimed it had successfully tested a nuclear warhead that could be mounted on a missile.

The nuclear programme has been accompanied by a series of ballistic missile launches, including from a submarine.

Given that Friday's test was the most powerful in terms of yield and that the time lapse from the previous test was shortened, "the North's nuclear capability is believed to have been sophisticated to a considerable degree and being developed at an increasingly faster pace," South Korean Foreign Minister Yun Byung Se told senior ministry officials.

The world must now "cautiously accept the reality" that the North could launch a nuclear attack by missile, said analyst Jeung Young Tae of the Korea Institute for National Unification (KINU), although the range of such a nuclear-tipped missile remained unclear.

The North's announcement of its test indicated they had tested the bomb that would arm their missile units, said Jeffrey Lewis, director of the East Asia Nonproliferation Program at Middlebury Institute of International Studies at Monterey.

"And that's a big deal," he wrote in an article entitled "North Korea's nuke program is way more sophisticated than you think", for the website of Foreign Policy magazine.

"In the past, we've treated North Korean nuclear tests as temper tantrums or political demonstrations."

The North is believed to have succeeded in making nuclear warheads small enough to arm Scud missiles to hit South Korea or Rodong middle-range missiles to attack Japan, Professor Yang Moo Jin of the University of North Korean Studies told AFP.

"But it has not yet completed the re-entry technology needed to develop an ICBM (inter-continental ballistic missile) that could hit Hawaii or the United States mainland," Yang said.

Given the enormous prestige and resources devoted to the nuclear programme, Yang doubts it would vanish even with regime change.

"Having witnessed with alarm what happened to Libya's Moamer Kadhafi and Iraq's Saddam Hussein, North Korea is deadly serious in believing its nuclear weapons are the only guarantee that can ensure its survival against an invasion by the US," he said.

"The nuclear arsenal gives Kim the halo as the country's top military commander and helps tighten his grip on power, both over the party and the military as well," Yang said.

"The North would never give up its nuclear arsenal unless it is guaranteed security, even if the Kim Jong-Un regime collapses and someone else took over."

But Kim is not a madman and is not about to launch a pre-emptive strike, Jeung of KINU told AFP.

"The sole purpose of Kim Jong Un's regime is survival of the regime but nothing more. So they must know very well that any pre-emptive nuclear strike will immediately prompt counter-attacks on North Korea, which will seriously jeopardise the regime.

"They may be reckless but they are not completely insane," Jeung said.

Some South Korean newspapers still flirted with doomsday scenarios.

Up to 235,000 would be killed if a nuclear blast of 10 kilotons occurred in Seoul, Yonhap news agency said, citing research in 2010 by the US think tank RAND Corp.

Top-selling Chosun daily also warned of "total destruction" and compared potential damage to the World War II bombing of Hiroshima and Nagasaki.