One patient was in critical condition and eight others were infected, most of them relatives of the first fatality, a 32-year-old herdsman, or local doctors, Xinhua news agency said.

Initial tests had shown that the herdsman's dead dog was the likely origin of the outbreak, Xinhua reported late Wednesday, quoting professor Wang Hu, director of the Qinghai disease control bureau.

Wang said it was likely that the dog died after eating a plague-infected marmot and that the man became infected when he was bitten by fleas while burying the dead dog. He died three days later.

"The first victim buried the dead dog without any protection. After he became infected, his relatives and neighbours were in close contact with him without taking any protective measures, leading to their infection," Wang was quoted as saying.

The World Health Organization says the bacteria which causes the plague is endemic in some rodents in the region, such as marmots.

Chinese health ministry experts quoted by Xinhua said the strict quarantine measures were proving effective and the outbreak was unlikely to spread further.

"There is no need to worry about the infection if you travel to Qinghai, not to speak of panic," professor Liang Wannian, deputy director of the ministry's emergency office, was quoted as saying.

The remote and mountainous area is sparsely populated, which is also helping to contain the outbreak.

Officials Thursday told Xinhua no one had escaped from the quarantine zone and that no new cases had been reported.

"Supplies in the area are guaranteed. It is not necessary for the locals to escape," said Dong Fukui, deputy head of the local prefecture government.

Residents of Ziketan contacted by AFP Wednesday said some people had tried to flee the quarantine zone, which covers an area of 3,500 square kilometres (1,400 square miles) centred on the town.

The WHO says pneumonic plague is the most virulent but least common form of plague. The mortality rate is high and patients can die 24 hours after infection.