SHANGHAI — China said Tuesday that four more people in the coastal part of the country had been infected with a new strain of bird flu, which is believed to have killed two Shanghai residents last month and left one person in critical condition.

The four new patients, ages 32 to 83, are hospitalized and critically ill, according to a government Web site that cited the authorities in the city of Nanjing, in Jiangsu Province northwest of Shanghai. The officials said laboratory tests had confirmed that all four were infected with a strain of bird flu identified as H7N9, which was not found in humans before the Shanghai cases.

The cases are troubling because there is no vaccine for the H7N9 strain and because another strain of bird flu, identified as H5N1, killed hundreds of people in Asia beginning in 2003.

The World Health Organization says that most H5N1 cases had involved contact with infected poultry. One of the four people in Nanjing infected with the H7N9 virus is a poultry butcher. Health authorities had previously reported that H7N9 could not easily be contracted by humans, and officials said that no one who had contact with the four infected patients had developed symptoms.