China on Sunday reported two human cases of highly pathogenic avian influenza H5N1 in southwestern city of Guiyang, the Ministry of Health announced.

A 21-year-old woman and a 31-year-old man, who developed symptoms on Feb. 2 and Feb 3, respectively, tested positive for H5N1 virus on Sunday, the ministry said, citing local health authorities.

"They are in critical condition and medical workers are carrying out emergency treatment," the ministry said in a statement. "No epidemiological connections have been found between the two cases."

An investigation did not find the two had contacts with fowls before they fell ill, it said.

People who had close contacts with the two patients have been put under medical observation, but none of them has been found ill so far.

The human-infected highly pathogenic avian influenza is an acute respiratory infectious disease mainly caused by the deadly H5N1 subtype virus. The main symptoms include high fever and pneumonia.

The human case of bird flu is mainly transmitted from poultry to humans and there are no reported outbreaks of sustained human-to-human transmission.