Chinese authorities have reported four new cases of a bird flu virus not previously seen in humans that has already killed two people in Shanghai.

The health bureau of China’s eastern Jiangsu province said three women and a retired man from different cities in the province were all in critical condition after contracting the H7N9 virus, in addition to three earlier cases confirmed in Anhui province and Shanghai, reports The Associated Press.

All four patients reported varying symptoms of dizziness, fever, coughing and respiratory problems. It is not immediately clear how the patients contracted the disease. Only one – a 45-year-old woman – appeared to have had daily contact with birds, Xinhua News Agency said.

The H7N9 virus has not been transmitted to humans before, according to National Health and Family Planning Commission, and there was no evidence yet that the disease is contagious.

Health authorities in Beijing vowed strengthen measures to monitor suspicious cases and prevent the spread of the disease, AP reports.

The World Health Organization expressed concern and said it would investigate whether the virus could spread more widely. “This is of concern,” Gregory Hartl, a WHO spokesman, said. “These are the first cases we’ve seen in human beings. We’re watching this very closely”.