BEIJING -- More than eight million acres of China's farmland is too polluted with heavy metals and other chemicals to use for growing food, a Cabinet official said Monday, highlighting a problem that is causing growing public concern.

The threat from pollution to China's food supply has been overshadowed by public alarm at smog and water contamination but is gaining attention following scandals over tainted rice and other crops. The government triggered complaints in February when it refused to release results of a nationwide survey of soil pollution, declaring them a state secret.

The figure given at a news conference by Wang Shiyuan, a deputy minister of the Ministry of Land and Resources, would be about two per cent of China's 337 million acres of arable land.

Some scientists have given higher estimates of as much as 60 million acres, or one-fifth of the total, though it is unclear how much of that would be too badly contaminated for farming.

The issue poses a dilemma for communist leaders who want to maximize food production but face public pressure to ensure safety after an avalanche of scandals over shoddy infant formula and other goods.

The explosive growth of Chinese industry, overuse of farm chemicals and lax environmental enforcement have left swathes of the countryside tainted by lead, cadmium, pesticides and other toxins.

Investigations by the Ministry of Environmental Protection have found "moderate to severe pollution" on 8.3 million acres, Wang said at a news conference.

"These areas cannot continue farming," Wang said. He did not say whether the information came from the national pollution survey.

Farmers already are prohibited from raising crops for human consumption in areas across China that are deemed too badly polluted. But tainted rice and other crops have made their way into the food supply.

The ruling Communist Party's latest five-year development plan, which runs through 2015, promises to reduce heavy metal pollution and clean up contaminated areas.

Wang said the government is working on a long-range plan and expects to spend several tens of billions of yuan (several billion dollars) a year on the effort. He gave no details but scientists say one possible approach is to plant trees or other vegetation that will absorb heavy metals from the soil but will not be consumed by humans.

Complaints by farmers about lead and other pollutants in their water supplies have led to protests against battery factories.

A key concern among scientists is cadmium, a carcinogenic metal that can cause kidney damage and other health problems and is absorbed by rice, the country's staple grain.

In May, authorities launched an investigation of rice mills in southern China after tests found almost half of supplies sold in Guangzhou, a major city, were contaminated with cadmium.

In February, the newspaper Nanfang Daily reported tens of thousands of tons of cadmium-tainted rice was sold to noodle makers in southern China from 2009 to this year. It said government inspectors declared it fit only for production of non-food goods such as industrial alcohol but a trader sold most of the rice to food processors anyway.