BEIJING - China's financial capital, Shanghai, activated an emergency plan Tuesday that calls for heightened monitoring of suspicious flu cases following the recent deaths of two men from a lesser-known strain of bird flu.

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Under the contingency plan, schools, hospitals and retirement facilities are to be on the alert for fevers, and administrators are to report to health authorities if there are more than five cases of flu in a week. Cases of severe pneumonia with unclear causes are to be reported daily by hospitals to health bureaus, up from the weekly norm. The plan also called for stronger monitoring of people who work at poultry farms or are exposed to birds.

The level-3 response plan, the second-lowest in a four-stage scale, reflects higher concern after the H7N9 bird flu virus led to the deaths of two men in Shanghai and seriously sickened a woman in the city of Chuzhou 360 kilometres west.

"The health bureau will take effective and powerful measures to prevent and control the disease, to make sure the flu epidemic is effectively guarded against and to safeguard the health of the city's residents," said Xu Jianguang, head of the Shanghai Health Bureau.

Since the new cases were reported Sunday, China's health agency has said specialists are investigating how the three became infected with the virus.

The H7N9 strain, so named for the combination of proteins on its surface, has previously been considered not easily transmitted to humans, unlike the more virulent H5N1 strain, which began ravaging poultry across Asia in 2003 and has since killed 360 people worldwide.

Health officials said this week there was no evidence 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.

Unverified reports on Chinese microblogs Tuesday said a fourth case of H7N9 surfaced in the eastern city of Nanjing. A post on the Sina Corporation's Weibo microblog appeared to be a photo of a patient record saying a 45-year-old woman in the city who works as a poultry butcher was critically ill and that a provincial disease centre confirmed a diagnosis of H7N9. The record indicated that the woman suffered from severe flu-like symptoms and was in intensive care, dependent on a respirator, at the Gulou Hospital.

Authorities at the hospital could not be reached while city and provincial offices said they did not know about the case or declined to comment.

Health authorities in Beijing also upped the capital's state of readiness, ordering hospitals to monitor for cases of bird flu and pneumonia without clear causes, the official Xinhua News Agency reported.

The announcements, as lacking in details as they are, show that the government is mildly more transparent in handling health crises than it was a decade ago during the SARS pneumonia epidemic. Then, as rumours circulated for weeks of an outbreak of an unidentified disease in southern Guangdong province, government silence contributed to the spread of the virus to many parts of China and to two dozen other countries.