Veterinarians expressed fears the disease could be related to outbreaks in neighbouring countries including Mongolia and Russia due to bird migration.

After tests conducted by the National Avian Influenza Reference Laboratory in Harbin on Wednesday, the Ministry of Agriculture informed the United Nations' Food and Agriculture Organisation yesterday that 133 chickens had died of H5N1 in Lhasa .

The ministry culled 2,608 chickens in the surrounding areas, said Zhang Zhongjun , the assistant representative in the FAO's Beijing office.

The outbreak was 'under control' as infected areas were sealed off in combination with a disinfection and vaccination campaign, the ministry told the FAO.

'We will monitor the situation, and are willing to offer help upon requests from the Chinese government,' Mr Zhang said.

The ministry also notified the World Organisation for Animal Health (OIE) on Wednesday, saying the outbreak was discovered on August 1, according to OIE spokeswoman Maria Zampaglione.

'They notified us it was a highly pathogenic virus of avian influenza, but we still don't know whether it is H5N1 or another strain,' she said.

Two H5N1 outbreaks occurred in geese and ducks in Xinjiang in June, following an outbreak among migratory birds in Qinghai in May.

Kazakh officials yesterday said bird flu had spread from a village near the Russian border in the north, to two villages to the west, and one in central Kazakhstan.

Guo Fusheng , the FAO's project regional co-ordinator on bird flu and a veterinarian specialising in infectious diseases, said he was worried the outbreaks could be related because of the migration of birds from north to south at this time of year.

'If bird flu from migratory birds spread to domestic poultry, the risk of the disease spreading to human beings will be higher, especially in Asia where poultry and human beings have close contact,' he said.

Of the many routes undertaken by migratory birds, one involves flying from Qinghai, via Yunnan and Tibet, to India, while another route would be from Siberia, via Mongolia and China, to South Asian countries, said Mr Guo.

Hong Kong does not import live chickens from Tibet, said Tsui Ming-tuen, chairman of the Hong Kong Live Poultry Wholesalers Association. But travel agents said they would avoid poultry on tour menus.