BEIJING - China faces a tough challenge if it wants to curb a regional outbreak of bird flu in the coming autumn and winter, said the nation's top veterinarian Yu Kangzhen on Wednesday.

The chance for a large-scale outbreak, however, remains quite slim, he said.

Yu made the remarks following a warning from the United Nations of a possible return of the deadly avian influenza epidemic, as a mutation of the H5N1 strain that can apparently sidestep defenses of existing vaccines was spreading in China and Vietnam.

"Since February 2009, China has reported zero bird flu outbreaks and the agriculture and health authorities are constantly on high alert of potential bird flu epidemics among poultry and humans," he told China Daily in a written statement.

But the virus has been detected sporadically among wild birds and in certain areas among farmed poultry, he admitted. "Also, mutations of the virus have been detected in the poultry."

In response, the Ministry of Agriculture ordered further heightened surveillance of the potentially deadly virus and made plans in the event of outbreaks, he said.

Since 2004, China has seen 50 cases of bird flu involving poultry on the mainland, with 31 in 2005 alone, official statistics show.

Authorities have set up a comprehensive mechanism to contain the virus, combining mass vaccination of poultry and timely culling of the infected.

"Based on results from surveillance, poultry on the mainland is basically safe from bird flu infections," Yu said.

So far this year, nearly 2.5 million samples have been tested for the virus and a mass vaccination against the virus will soon kick off on poultry farms nationwide, he added.

As precautions, quarantine efforts, particularly on the borders, will be further beefed up to avert any imported cases, he said.

Previously, the UN Food and Agriculture Organization said bird migrations over the past two years had brought H5N1 to countries that had been virus-free for several years, including Israel, Bulgaria, Romania, Nepal and Mongolia.

Worse, the mutant strains posed unpredictable risks to human health, the organization warned.

Shu Yuelong, director of the National Influenza Center of the Chinese Center for Disease Control and Prevention, told China Daily that the virus actually has been constantly mutating among poultry.

"But so far no evidence has showed that the mutant strains could jump easier from birds to humans," he said.

"The possibility of major outbreaks of human bird flu can be basically ruled out and there is no need to worry," he added.

Since 2003, H5N1 has infected 565 people worldwide, killing 331, according to the World Health Organization. A 6-year-old Cambodian girl died on Aug 14 from bird flu, the eighth person to die from H5N1 avian influenza this year in the country.