By David Stanway and Niu Shuping

BEIJING (Reuters) - China's state media are working overtime to persuade the public that genetically modified food is safe, apparently softening up the population for a policy switch to allow the sale of such food to ensure its 1.35 billion people have enough to eat.

In the past 30 years, China's urban population has jumped to about 700 million from under 200 million, driving up demand for meat and staples such as rice that scientists say only GMO can satisfy.

Imported GMO soybeans are already used as feed for animals but winning acceptance for the more widespread use of GMO may be a hard sell in a country frequently in the grip of food scares -- just this year over baby milk powder and chemicals in chickens, for instance.

GMO food faces opposition even at the top levels of Chinese bureaucracy, with a senior national security official likening it to opium.

But state media is taking up the fight: on Monday, Communist Party mouthpiece the People's Daily rejected rumors that eating GMO food could alter human DNA, and news agency Xinhua ran an investigation last week debunking tales that GMO corn consumption had reduced sperm counts.

Zhang Qifa, known as China's "father of GMO rice", recently criticized the Ministry of Agriculture for refusing to approve strains that have cost billions of yuan in research over the past decade.

Beijing granted safety certificates for its first genetically modified rice in 2009 but has so far refused to authorize commercial production until the public is onside.

The certificate for Zhang's pest-resistant "Bt" rice will expire next year, meaning researchers have to reapply, a process that could take years.

"Right now, China's GMO rice production has ground to a halt ... I personally think we have missed opportunities to develop," Zhang said, adding that GMO commercialization wasn't a matter for the public and should begin without delay.

Huang Dafang, a researcher with the Biotechnology Research Institute of the Chinese Academy of Agricultural Sciences, was unimpressed with the media campaign. "We have not seen any signs of progress, only the continuation of the debate."

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Scientists have been at pains to show that GMO is already part of the food chain: China is the world's top importer of GMO soybeans, used as feed, and also imports GMO corn from the United States and elsewhere.

The U.S. Department of Agriculture has forecast China's rice imports would reach a record high of 3.4 million tons in 2013/14 and researchers say China is facing a growing food gap that can only be properly addressed through the use of GMO.

But while policymakers have expressed optimism about GMO crops and scientists have long urged the government to allow new strains of GMO rice, Beijing will not move until it is sure the risks are minimal and that, crucially, the public is behind it.

A NEW KIND OF OPIUM

The debate hasn't been entirely one-way, with influential researchers still urging caution, especially when it comes to staples like rice and wheat.

"Many have said there are no risks to GMO food but the risks may not even be discovered in three or five years but actually over three to five generations," said Jiang Changyun, research director at the Industrial Development Research Institute, who wants the government to improve food labeling so that people can decide themselves whether to eat GMO or not.

The debate has moved into the realms of national security, with Peng Guangqian, deputy secretary-general of the National Security Policy Committee, likening GMO food in August to a new kind of opium being forced upon China by Western companies.

Writing in Global Times, a tabloid backed by the People's Daily, Peng said companies such as Monsanto and Dupont were dumping GMO products on China.

Wang Xiaoyu, an official at the Heilongjiang Soybean Association, said GMO soyoil consumed in southern parts of the country was linked to high cancer rates.

However, another worry, he conceded, was that imports of cheap GMO soy had led to a fall in local production, since many planters were unable to compete.

Huang of the Biotechnology Research Institute complained that the scientific debate had been hijacked.

"GMO is a scientific matter and should not be debated at the social level. If China's Three Gorges dam and nuclear power were decided by public debate, neither would have been established," he said.

(Editing by Alan Raybould)