China plans to draw a line in the sand to ensure a minimum 120 million hectares (463,325 sq miles) of arable land remains to feed its people, a government minister said on Thursday.

China, which has a fifth of the world's population but only 7 percent of its arable land, has been losing farmland to factories, houses and roads as its economy booms. It became a net importer of food in 2004, increasing Beijing's worries over how to maintain food security.

"The interests of the state come above all else, as do those of the people. The 1.8 billion mu (120 million hectares) of arable land red line is the high voltage line which nobody can touch," Minister of Land and Resources Xu Shaoshi told a news conference, according to a transcript on the government Web site (www.gov.cn).

"Anyone who approaches the red line will not get off lightly," Xu added. "We must guard the 1.8 billion mu red line, and we most definitely will."

About 80 percent of all cases of illegal use of land -- which in China is ultimately owned by the state -- involves houses built in the countryside, he said.

Most of those cases involved some government department, added Gan Zangchun, deputy chief inspector of land.

Central government planners have reiterated the need to protect farmland, but to little effect as urban sprawl takes over China's fertile flat lands.

Agriculture Minister Sun Zhengcai was quoted this week in state media as saying it would be hard to reverse the trend of shrinking farmland.

In the last 10 years, the area of land under cultivation had fallen to 1.827 billion mu from 1.951 billion mu, he said.

Although development is prohibited for land zoned as farmland, local officials often simply change the zoning before releasing it to developers. Land is often also seized by companies or local governments from farmers with little or no compensation provided, leading to sometimes violent confrontations in the countryside.

Still, Xu said that protecting farmland did not mean there would not be enough space for other developments.

There was at least 3.9 billion mu (260 million hectares) of land which could be used, but the crux was that good arable land should not be built on, he said.

"What is crucial is that construction should, as much as possible, not happen on farmland, and if it must then it should use a little land as possible," Xu said.

Xu did not mention the problem of desertification which the government says is the main environmental challenge holding back China's sustainable development.

Deserts, which cover a fifth of China, are spreading on the upper reaches of the Yellow River, on the Qinghai-Tibet plateau and parts of Inner Mongolia and Gansu.