A third man died of pneumonic plague in northwest China in an outbreak that prompted authorities to lock down a town where about a dozen people were infected with the highly contagious deadly lung disease, the state news agency said in Beijing on Monday.

The World Health Organization office in China said it was in close contact with Chinese health authorities and that measures taken so far to treat and quarantine sickened people were appropriate.

The 64-year-old man, named Danzhi, died in Ziketan in northwestern Qinghai province, where two other men died of the highly contagious lung disease in recent days.

The second man, Danzin, 37, died on Sunday, the official Xinhua News Agency said. Danzin was a neighbour of the first person who died, a 32-year-old herdsman whose name was not released.

Another nine people, mostly relatives of the first deceased man, were infected and undergoing isolated treatment in hospital, Xinhua said in a report late Sunday.

The town of 10,000 people has been sealed off, and a team of experts was sent to the area, the local health bureau said Sunday, warning that anyone with a cough or fever who visited the town since mid-July should seek treatment at a hospital.

Pneumonic plague is spread through the air and can be passed from person to person through coughing, according to the WHO. It is caused by the same bacteria that occurs in bubonic plague — the so-called Black Death that killed an estimated 25 million people in Europe during the Middle Ages.

Bubonic plague is usually transmitted by flea bite and can be treated with antibiotics if diagnosed early. Pneumonic plague is one of the deadliest infectious diseases, capable of killing humans within 24 hours of infection, according to the WHO.