BEIJING - China has reportedly asked local authorities to be on guard against a resurgence of bird flu with the onset of the autumn migration season.

"Autumn and winter are the prime period for bird flu outbreaks," said Yin Chengjie, vice-minister of agriculture, told the official Xinhua news agency, adding that officials "should not underestimate the difficulties in virus control."

Yin said Thursday the situation in China remained serious and warned there was still much to be done to prevent the spread of the virus, as the annual migration risked sparking new outbreaks.

On Wednesday, China's health ministry announced that a new outbreak of bird flu had killed about 1,000 poultry in northern China, the second such case in a week.

Twenty-one people in China have contracted bird flu and 14 of them have died, according to official figures. The most recent fatality occurred in July in the western region of Xinjiang.

More than 220 people have contracted the virus in Asia since the end of 2003 and 139 have died.

China confirmed in August that its first human bird flu victim died in late 2003, two years earlier than previously reported.