Two Shanghai men have died from a lesser-known type of bird flu in the first known human deaths from the strain, and Chinese authorities said it wasn't clear how they were infected but there was no evidence of human-to-human transmission.

A third person, a woman in the nearby province of Anhui, also contracted the H7N9 strain and was in critical condition, China's National Health and Family Planning Commission said in a report on its website Sunday.

There was no sign that any of the three, who were infected over the past two months, had contracted the disease from each other, and no sign of infection in the 88 people who had closest contact with them, the medical agency said.

H7N9 bird flu is considered a low pathogenic strain that cannot easily be contracted by humans. The overwhelming majority of human deaths from bird flu have been caused by the more virulent H5N1, which decimated poultry stocks across Asia in 2003.

The World Health Organization is "closely monitoring the situation" in China, regional agency spokesman Timothy O'Leary said in Manila.

"There is apparently no evidence of human-to-human transmission, and transmission of the virus appears to be inefficient, therefore the risk to public health would appear to be low," O'Leary said.

The 87-year-old victim became ill on Feb. 19 and died on Feb 27. The other man, 27, became ill on Feb. 27 and died on March 4, the Chinese health commission said. A 35-year-old woman in the Anhui city of Chuzhou became ill on March 9 and is being treated.

The Chinese Center for Disease Control and Prevention conducted tests and confirmed Saturday that all three cases were H7N9, the health commission said.

Scientists have been closely monitoring the H5N1 strain of the virus, fearing that it could mutate into a form that spreads easily among people, potentially sparking a pandemic. So far, most human cases have been connected to contact with infected birds.