BEIJING - China reported Tuesday that four more people in one province were seriously sickened by a bird flu virus new to humans while cities along the eastern seaboard stepped up public health measures to guard against a disease that has already caused two deaths.

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The health bureau of eastern Jiangsu province said in a notice on its website that three women, aged 45, 48 and 32, and an 83-year-old retired man, from different cities in the province, were all critically ill with the H7N9 virus, a diagnosis confirmed by the provincial disease prevention centre. These latest cases are the second batch to be confirmed after three in Anhui province and nearby Shanghai on Sunday.

The H7N9 strain, so named for the combination of proteins on its surface, has previously not been a problem in humans. That compares to the more virulent H5N1 strain, which began ravaging poultry across Asia in 2003 and has since killed 360 people worldwide, mostly after close contact with infected birds.

The reports of the new cases likely suggested that authorities were coming to grips with the virus. With the health agency having identified the first known infections on Sunday, authorities were likely taking a closer look at other severe flu cases.

"When you don't look, you don't find them, but when you look, you'll find," said Dr. Ray Yip, a public health expert who heads the Bill and Melinda Gates Foundation in China.

"A lot of people get severe respiratory conditions, pneumonias, so you usually don't test them. Now all of a sudden you get this new reported strain of flu and so people are going to submit more samples to test, (so) you're more likely to see more cases," Yip said.

All the patients have been sick since about March 19, when they had fevers, coughs and other flu-like symptoms, the statement said. Their conditions worsened over periods of time ranging from a week to 11 days such that they were transferred to intensive care units in the provincial capital, Nanjing.

Based on the bureau's statement, only one of the patients appeared to come into daily contact with birds - the 45-year-old woman, who was described as a poultry butcher. The four cases did not appear to be connected, and people who have had close contact with the patients have not reported having fevers or respiratory problems, it said.

The provincial health bureau said it was strengthening measures to monitor suspicious cases and urged the public to stay calm, joining Beijing and China's financial capital, Shanghai, in rolling out new steps to respond to the relatively unknown virus.

The latest cases follow the three earlier ones reported Sunday, including two men who died in Shanghai, resulting in the city activating an emergency plan that calls for heightened monitoring of suspicious flu cases. 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 in Shanghai and seriously sickened a woman in the city of Chuzhou 360 kilometres (230 miles) 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.

Health officials said this week there was no evidence that any of the three earlier cases, 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.

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.

Scientists are closely monitoring these viruses for fear they could mutate into a strain that easy spreads among people, but there's no evidence of that occurring.