The human death toll from the bird flu has risen to 12 and the World Health Organization says China needs to give more information about the disease's spread as more provinces announce outbreaks in chickens.

An 18-year-old man became the ninth person in Vietnam to die after contracting the H5N1 virus, which causes bird flu. Of the 10 confirmed human bird flu cases in Vietnam, only one person is still alive.

Officials in Thailand also said Monday that the disease has killed a 58-year-old woman, bringing the number of deaths there to three. More than a dozen other people in Thailand are suspected bird flu cases.

The virus has spread to poultry in at least 10 Asian countries, including a third of China's provinces. So far, the only human cases this year have been in Vietnam and Thailand, but U.N. experts say it is possible that human cases elsewhere have gone undetected.

Roy Wadia, a spokesman for the World Health Organization in Beijing, says China needs to give more information about how it is tracking the disease in chickens. "We would like a quick assessment of the surveillance systems, especially in the high-risk areas, which have large poultry populations," he said. "Secondly, the vaccination program that they have for these animal and birds."

Another WHO official said the spread of the disease in China raises many questions because it appears to be skipping around the country. He said that may mean poorer provinces in China's central region lack the medical infrastructure to quickly find and report outbreaks.

Experts also say there is a possibility that human cases are going undetected in Indonesia, which has been slow to give details about the outbreak there among poultry. Indonesia's poverty and vast stretch of islands makes it harder to track disease outbreaks.

Although the WHO has said that two Vietnamese women may have caught the bird flu from an infected relative, scientists say human-to-human transmission seems quite limited. They also say the virus does not seem to spread efficiently among humans.

The H5N1 virus was first detected when it crossed from chickens to humans in Hong Kong in 1997. It infected 18 people, killing six. This is the largest outbreak ever recorded and more than 45 million chickens in Asia reportedly have been culled to try to stop the spread of the virus.