Concerns over the sale of the land to the Korean firm have been further heightened after the city assembly of the South Korean town of Masan declared in 2005 that June 19 would be Daemado Day - the Korean name for the islands - and claimed sovereignty over the territory.

The declaration is clearly a tit-for-tat manoeuvre in the ongoing rival claims to the Dokdo islands, which are claimed by Japan but controlled by South Korea, but has aroused fears among many Japanese that more of the nation's outlying islands could be claimed and seized by covetous neighbours.

A special committee of the ruling Liberal Democratic Party tasked with monitoring security and land legislation has begun studying whether regulations are required to make sure that plots of land that are considered critical to Japan's national security are not sold to foreign entities.

"It is important to regularly obtain land registration data from legal affairs bureaus and to exercise caution and conduct adequate surveillance," Itsunori Onodera, the defence minister, told the committee, specifically citing the Tsushima Island case.

An official of the ministry briefed the committee on the case, with one of the politicians quoted subsequently as saying: "It will be a serious matter unless the state takes steps."

That assessment is shared by Go Ito, a professor of international relations at Tokyo's Meiji University, who said it would be impossible for a Japanese firm to buy a plot of land alongside a Chinese military facility, for example. "Any purchase of land should ideally be based on goodwill and simply be a commercial transaction, but there are absolutely no guarantees that will happen every time," he told the .

"That is why other countries already have restrictions on what land or property can be bought by foreign nationals," he said. "And, to me, that makes complete sense.

"I would hope that other countries would not see this as a case of nationalism, protectionism or any form of xenophobia, but merely a sensible and reasonable effort to guarantee land use," he said. But he added that there are probably some sectors of society and the media in China and South Korea, in particular, that might use any new law restricting purchases to criticise Japan.

"That is maybe inevitable, given the poor state of relations between Japan and those two countries at the moment."

Others are concerned that foreign investors are snapping up parcels of land - an issue highlighted by magazine in late 2012 under the alarmist headline "China will buy up Japan". An underlying fear is that the private companies carrying out the transactions might only be front companies for national governments interested in gaining a foothold in a foreign territory.

In 2010, residents of Nagoya opposed the construction of a new Chinese consulate in the city, while there have been reports of speculators from Shanghai and Beijing buying blocks of cheap apartments and renting them to Chinese students.

National security remains the government's biggest preoccupation, however, after a Chinese consortium reportedly tried to buy an uninhabited island in the straits between Honshu and Shikoku, close to naval bases used by the Japanese and US navies.