Four people in Jiangsu province are fighting for their lives after it was confirmed they were infected with H7N9 bird flu.
The news follows the deaths of two men in Shanghai. A woman from Anhui is also critically ill.
Public hospitals in Hong Kong stepped up measures to spot suspected cases and sampling at poultry farms has increased.
The Department of Health advised travellers returning from Jiangsu, Shanghai and Anhui with respiratory symptoms to wear masks and seek medical attention.
Health and agricultural authorities in Beijing, Guangdong and Shandong have also announced precautionary measures, despite not reporting any confirmed cases so far.
Google Map: reported cases on H7N9 human infections in eastern China
Jiangsu's health department said last night the four new cases developed symptoms between March 19 and 21.
It said a 45-year-old woman engaged in slaughtering work in a farmers' market in Nanjing sought treatment for fever and a cough and was admitted to intensive care last Wednesday.
A 48-year-old woman from Suqian, an 83-year-old man in Suzhou and a 32-year-old woman in Wuxi all showed symptoms two weeks ago and were diagnosed as having contracted H7N9. Those diagnoses were confirmed by laboratory tests yesterday.
The department said all four were in critical condition, but 167 people with whom they had been in close contact had not developed fevers or symptoms of respiratory illness.
All seven confirmed cases have occurred in the Yangtze River Delta. Doctors have yet to find out how the virus was transmitted. "So far no epidemiological connection has been found among the four cases [in Jiangsu]," the province's health department said.
H7N9 bird flu is considered a low-pathogenic strain that cannot easily be contracted by humans. The more common cause of human deaths from bird flu in Asia is the H5N1 strain.
On Sunday, the National Health and Family Planning Commission announced the first known human cases of H7N9 bird flu - two men aged 87 and 27 who died in Shanghai and a 35-year-old woman from Anhui who was being treated in Jiangsu.
The DNA sequence of the H7N9 virus in the first three patients was shared online on an international scientists' exchange platform yesterday.
View H7N9 map in a larger map
Hong Kong's health authorities confirmed the DNA was from different bird viruses, but had not mixed with human or pig flu viruses.
Centre for Health Protection controller Dr Leung Ting-hung said: "If highly pathogenic avian viral DNA mixes with human influenza viral DNA, a serious situation of human-to-human transmission may happen. This is what we're most worried about."
Wu Fan, director of the Shanghai Centre for Disease Control and Prevention, said yesterday it was still unclear whether the two local cases were of humans contracting bird flu or the bird flu mutating to a human flu virus.
Although the two H7N9 deaths both occurred at the Shanghai No5 Hospital in the city's Minhang district, hospital infection had been ruled out.
The death of the 27-year-old, a butcher who according to his family had no close contact with poultry, sparked panic about sick pigs. More than 10,000 dead pigs floated from neighbouring Zhejiang province to Shanghai's Huangpu River last month.
Mainland agricultural authorities conducted tests on 34 samples from the dead pigs and did not find any strains of bird flu.
The Anhui woman, a housewife, cooked a chicken slaughtered at a wet market a week before developing symptoms.
Additional reporting by Daniel Ren and Danny Mok