Pyongyang's nuclear defiance is prompting fears of a backlash against North Koreans living in Japan, many of whom already face daily discrimination.
Since North Korea said on Monday it had carried out a nuclear test, schools and other facilities run by the pro-Pyongyang General Association of Korean Residents in Japan (Chongryon) have received threatening calls, sparking worries of worse to come.
"There have been no reports of serious incidents so far. But in the past when the DPRK (North Korea) did something conspicuous, some radical people resorted to violence against students at our schools," said one Chongryon official who declined to be identified.
"That's why we are worried."
Reflecting such concerns, police guarded the entrances to "Korea Town" in Kawasaki City, near Tokyo, on Friday, and residents, many North Korean, shied away from reporters.
About 600,000 ethnic Koreans live in Japan, many of them descended from the 2 million Koreans brought to Japan as forced labour during Tokyo's 1910-1945 colonisation of the peninsula.
Of those, about 80,000 are pro-North, another 220,000 support the South, and the rest back neither country, said Pyon Jin-il, himself an ethnic Korean and an expert on North Korea.
Japan granted Koreans Japanese nationality during its colonial rule, but stripped them of the status in 1952 and deprived them of many benefits enjoyed by Japanese citizens.
Many girls attending Chongryon-run schools wear traditional Korean "chima chogori" uniforms and hence are easily singled out.
But most Koreans, born and raised in Japan, differ little from their Japanese neighbours on the surface, speaking fluent Japanese and often marrying Japanese spouses.
Chongryon officials, though, worry that Pyongyang's nuclear brinkmanship and Japan's stern reaction could fan mistrust.
"The greater fear is that the heated rhetoric from both Japanese political leaders and Japan's media ... will escalate to the point where it will be extremely difficult for diplomacy to calm the situation," Kim Jong-ui, a Chongryon official in Osaka, western Japan, was quoted as saying in the Japan Times.
"This could result in further discrimination, including physical harassment by Japanese of Koreans in Japan."
Some North Koreans living in Japan are likely to feel the pinch of sanctions ordered by Japan's cabinet on Friday against Pyongyang, including a six-month ban on all imports.
"They're the ones who will get hit directly because they act as middlemen for many transactions," said Lee Jong-won, an international politics professor at Rikkyo University.
Job opportunities for pro-Pyongyang Koreans are limited largely to firms run by members of their community, such as the pachinko pinball parlours, night clubs, barbecue restaurants and trading houses specialising in business with the North.
Thousands of Korean residents already become naturalised Japanese citizens each year, opting for assimilation.
Chongryon's membership also took a hit after Pyongyang admitted in 2002 that it had abducted 13 Japanese in the 1970s and 1980s to help train spies. That trend could speed up now.
"If North Korea raises military tensions further, it will spark more harassment against Korean residents and could lead to Chongryon's collapse," Pyon said.
Chongryon officials shy away from public criticism of North Korea, but privately some members blame their ancestral homeland.
"We feel ashamed of what our fatherland's government has done," said a 49-year-old Chongryon member who declined to be identified. Like many, though, he feels he has no option but to stay in Chongryon because of relatives in the North.
The group is Pyongyang's de facto embassy in Japan, since the two countries have no formal diplomatic relations.
Tokyo's sanctions will bite some North Koreans in Japan, but will have less impact on the North's struggling economy.
Two-way trade has already dwindled and Chongryon is not the big source of funds for the North it once was.
"During the heyday of pachinko (pinball) business in the 1980s, a lot of cash was siphoned into North Korea," Pyon said.
"But that's a thing of the past."