China believes the mystery illness that has killed 19 farmers in western China is streptococcus suis, a disease common in pigs.
China's health ministry said people had contracted the disease by slaughtering and processing infected pigs.
Sixty-seven confirmed cases and 13 suspected cases of the disease had been reported as of noon on Sunday, it said.
The WHO noted the "disconcertingly high mortality rate" and said it was keeping a close watch on the situation.
"According to research and lab test results, experts believe the disease is caused by streptococcus suis," the health ministry said in a statement.
The symptoms include high fever, nausea and haemorrhaging.
The latest infections were spread throughout 75 villages and 40 towns near the cities of Ziyang and Neijiang, the health ministry said.
The number of people infected with the illness has risen steadily as health officials searched through remote villages in the province for people with symptoms.
"We are looking at not just a bacteria being active in one herd of pigs, but over a fairly wide area," said Bob Dietz, a spokesman for the World Health Organization's regional office in Manila.
"We see this as a serious situation which bears close monitoring," Mr Dietz said.
In Hong Kong, health officials warned hospitals to look out for similar symptoms and banned the import of pork from Sichuan.
Correspondents say the territory has been wary of diseases spreading from mainland China since the outbreak of the acute respiratory disease Sars, which killed nearly 300 people there in 2003.