A bus driver in southern China who contracted the bird flu virus died on Saturday, health authorities said, in the nation's first reported human case of the deadly disease in 18 months.

The man, surnamed Chen, died in Shenzhen -- a boomtown that borders Hong Kong where thousands of chickens have already been culled after three birds tested positive for the H5N1 avian influenza virus in mid-December.

He developed a fever on December 21 and was taken to hospital four days later, and diagnosed with severe pneumonia, said the health department in Shenzhen, a city of more than 10 million people.

The 39-year-old then tested positive for the H5N1 virus, the department said, adding he had apparently had no direct contact with poultry in the month before he was taken ill, nor had he left the city.

The H5N1 virus is fatal in humans in about 60 percent of cases.

However, it does not pass easily from human to human, and the World Health Organization says it has never identified a "sustained human-to-human spread" of the virus since it re-emerged in 2003.

The health department in Guangdong province, where Shenzhen is located, announced Saturday that the bus driver died after his lung, heart and liver functions deteriorated.

"So far, 120 people who have had close contact with him have not presented abnormal symptoms," it said in a statement.

An official at the Shenzhen agriculture and fisheries bureau, surnamed Jiang, told AFP the bus driver had had no contact with birds.

"So far, we have not received any reports of any birds being infected," he said.

"It is unclear where the patient got the flu from. We will not make any plans to kill domestic birds unless we know that was the source, or if there is any sign of birds being infected."