Two reports released in the space of a few hours on Tuesdaymade the alarming claim that North Korea had successfully developed a miniaturized nuclear warhead that can be attached to a ballistic missile - a technical leap that most analysts had previously believed was still beyond the regime's scientists.

To further stoke regional tensions, Pyongyang announced early on Wednesday that it was "carefully examining" a plan to strike US Pacific territory of Guam with its long-range ballistic missiles.

That declaration came shortly after US President Donald Trump threatened that any North Korean attack would be met with "fire and fury" -an uncharacteristically extreme tone from a US president. 

The rate at which the security situation in Northeast Asia is deteriorating is causing alarm in Japan, which is home to numerous US military installations and is well within range of North Korean missiles.

On Tuesday, Tokyo released its annual defense white paper, reporting that North Korea's nuclear weapons and missile programs have attained "a new level of threat" toward Japan and the international community. The previous report stated that the programs posed "serious and imminent threats."

'Considerable advances'

"It is conceivable that North Korea's nuclear weapons program has already made considerable advances and it is possible that North Korea has already achieved the miniaturization of nuclear weapons and has acquired nuclear warheads," the report said.

"Since last year, when it forcibly implemented two nuclear tests and more than 20 ballistic missile launches, the security threats have entered a new stage," the 563-page reported continued.

The study went substantially beyond what analysts and intelligence officials had previously believed North Korea is capable of. Its assertions were quickly corroborated by a report released by the US daily the Washington Post quoting a confidential US assessment that Pyongyang had successfully miniaturized a nuclear warhead.

The paper reported that the Defense Intelligence Agency concluded in July that North Korea had crossed a key threshold in its quest to become a full nuclear power. Another assessment calculates that North Korean leader Kim Jong Un has an arsenal of as many as 60 nuclear weapons at his disposal.

Garren Mulloy, an associate professor of international relations at Japan's Daito Bunka University, said these latest advances are cause for serious concern.

"It is not just the speed at which they have made these developments - which alone have caught the international community off-guard - but the way that they have not funneled all their resources into their nuclear miniaturization program," he told DW.

Cause for alarm

The expert added that North Korea was making "great strides" in diversifying their delivery systems. The tried-and-tested launch of missiles from fixed launch pads has been perfected, but such missiles are vulnerable to pre-emptive strikes, Mulloy said.

So in tandem with the nuclear program, the North has developed and deployed mobile tractor-erector-launcher (TEL) units that can move around the country and are far more difficult to detect, he pointed out.

The third prong of the North's long-range weapons program is the submarine-launched ballistic missile (SLBM) that has not yet been completed, but has seen some rapid advances in the last 18 months.

Media coverage in Japan has triggered alarm in some parts of the country, particularly areas facing the Korean Peninsula across the Sea of Japan. A number of towns have introduced periodic drills simulating a missile strike nearby, with local residents encouraged to heed warning sirens and take cover if they are unable to evacuate.

The government has also recently updated information on the Cabinet Secretariat's Civil Protection website about measures that citizens should take in the event of a missile attack. It says that people should take cover inside sturdy buildings or underground shopping facilities, while anyone caught in the open should lie flat on the ground and behind shelter, if possible. People indoors are also being told to stay away from windows.

School warnings

In April, an international school in Yokohama issued an alert to its staff, cautioning, "As a result of heightened tensions between North Korea and the United States, there has been a precautionary warning by the governmentof Japan that North Korea may launch a missile and it may be directed at Japan."

"In this case, the government will sound an alarm and there will be an emergency announcement made over the PA system by the school head/designee for teachers to take the students that you are supervising to the auditorium," it said. "It will be a tight squeeze, but everyone needs to shelter there until the all clear is given."

Elsewhere, a company that markets bomb shelters and air-purifying equipment designed to keep radioactivity and toxic chemicals at bay says it is fielding as many as 30 inquiries for its products every day, up from around five a year ago.

And customers are not being put off by the 25 million yen (193,499 euros) price tag for bomb shelters, although many are expressing concern at the long waiting list.

"When we watch the news, it's always very worrying," said Kanako Hosomura, a housewife from Yokohama, south of Tokyo. "A few years ago we only heard when North Korea did a nuclear test or a missile launch, but now it seems that every day there is something about how they have made some advance that makes their missiles bigger or better."

"Everyone knows that their missiles can hit anywhere in Japan now and the big US naval base at Yokosuka is less than 40 kilometers from here," she said. "I fear that if Kim feels that his regime is going to collapse, then he might just fire his missiles anyway."

Pre-emptive strike?

The same concerns are also clearly at the forefront of the Japanese government's thinking, with a significant faction within the ruling Liberal Democratic Party pressing for Japan to formally announce that it retains the self-defense principal of being able to carry out a pre-emptive strike against an enemy that is preparing to attack Japan.

Prime Minister Shinzo Abe said on Sunday that he intends to carry out a study in the near futureinto the possibility of equipping Japan with the military ability to carry out such a first strike, although analysts here say the legal ability to do just that already exists in an unwritten form.

"My gut feeling would be that the LDP would not formally move in that direction, although the party has consistently since the start of the Cold War maintained that Japan has the right to strike if it detects an 'imminent, direct and existential threat' to Japan," said Mulloy.

"But when it comes to a nuclear threat against Japan, I think Tokyo would prefer the US to do that and that it would provide support, as we saw with Japanese fighters escorting US bombers on recent exercises," he said.