Climb down some winding steps in Mr Seiichiro Nishimoto's home in Osaka, Japan, and one reaches a rather unusual feature: a nuclear bunker built into the basement.

The air-tight structure features an anti-blast door, radiation-blocking air purifiers, a Geiger counter to measure radiation, a television set, gas masks and other emergency supplies.

As the chief executive of Shelter, a company that produces custom-built nuclear shelters, Mr Nishimoto has built this bunker as a model room to show his customers.

Sales of nuclear shelters and anti-radiation air purifiers have surged in Japan in recent weeks, according to a Reuters report, as neighbouring North Korea presses ahead with missile launches and nuclear tests.

Concerns about a possible gas attack grew in Japan after Japanese Prime Minister Shinzo Abe told a Parliament session this month that North Korea may have the capacity to deliver missiles equipped with sarin nerve gas.

Tokyo has also urged local governments to hold evacuation drills in case of a possible missile attack, heightening a sense of urgency among the public.