China could face problems trying to control bird flu this spring due to abnormally high seasonal temperatures, the country´s vice agriculture minister said.

"First, the spring season is usually prone to bird flu. Second, as the average temperature in north and central China is higher than the past, migrant birds may stay longer within the borders of China," Yin Chengjie was quoted by Xinhua news agency as saying.

"We have successfully stopped the spread of bird flu from migrant birds in Qinghai and Tibet to human beings and domestic poultry. Now, we need to intensify monitoring of bird migration and be on high alert," he said in comments seen on Tuesday.

An expected increase in the transport of animals over the traditional Lunar New Year period, which starts this weekend, means the government must step up inspection and quarantine work, Yin added.

The H5N1 flu virus remains mainly a virus of birds, but experts fear it could change into a form easily transmitted from person to person and sweep the world, killing millions within weeks or months.

So far, most human cases can be traced to direct or indirect contact with infected birds.

China has not reported a poultry outbreak since Sept. 20 last year, although the health ministry in January confirmed that a man in the eastern province of Anhui had contracted bird flu but subsequently recovered.