The virus has so far killed 22 people in China and sickened 108, CNN reported Wednesday.
The virus is "unusually dangerous," Keiji Fukuda, an assistant director-general of the World Health Organization, said during a news conference in Beijing.
"This is definitely one of the most lethal influenza viruses that we have seen so far," Fukuda said.
He added the disease appears to be transmitted more easily than the H5N1 virus that killed 332 people between 2004 and 2007.
The first case of H7N9 has now been reported outside of China. A 53-year-old Taiwanese man who had worked in eastern China was confirmed Wednesday to have contracted the virus.
The Taiwanese Centers for Disease Control described the man's condition as serious. Center officials said he had not been exposed to any birds or poultry during his stay in Jingsu province and had not eaten any undercooked poultry or eggs.
Authorities say there is no evidence the disease can be passed between humans.
Researchers saw a "dramatic slowdown" in the number of people in Shanghai who contracted the disease after the city closed its live poultry markets, said Anne Kelso, the director of a research center that collaborates with the World Health Organization.