China has not provided international health agencies with samples of bird flu viruses found in the country since 2004, but is putting in place procedures to do so, a senior official said on Tuesday.

Scientific analyses of bird flu samples are important because they help experts trace the evolution of viruses and the geographical spread of any particular strain.

"When viral strains cross international borders, special protocols are needed and we are working to complete those," vice director of the Ministry of Agriculture´s veterinary department, Li Jinxiang, told a news briefing.

Li said Beijing wanted "to conform to WHO (World Health Organization) standards for international transfers".

Though China has not submitted bird flu samples since late 2004, it has regularly reported results of its laboratory tests, including genetic information, to international bodies, Li added.

Health experts fear the H5N1 bird flu virus, which has killed at least 140 people worldwide since late 2003, will mutate into a form that can pass easy among humans, sparking a pandemic that could kill millions.

China was gearing up for the autumn poultry vaccination effort, Li said, which would also include monitoring for signs of vaccine-resistant strains.

China has reported several cases of bird flu in humans in areas without known outbreaks among poultry, opening the question whether poultry outbreaks are going undetected or are somehow masked by vaccines.

Nearly 5 billion poultry were vaccinated in the first six months of 2006, Li said. Samples taken during that period exceeded those taken in the whole of 2006, he said.

But he conceded that there were still some gaps in coverage.

"We have hit some difficulties in combatting bird flu," Li said, adding that infections among wild birds and waterfowl might have allowed the virus to proliferate.

Vaccination drives had also failed to penetrate some remote areas, he said.