The warning came Friday as Cambodia's Prey Veng Province bordering southern Vietnam reported two human infections of H5N1, including one death, and several suspected cases.
Cambodian authorities Thursday reported the death of an 11-year old girl and began testing 12 people who had come into contact with her. Her father has also tested positive.
The World Health Organization said it was in close contact with the Cambodian authorities about the situation. It has expressed concern as the father's result raised fears of human-to-human transmission.
Dr Nguyen Vu Thuong, deputy director of the Pasteur Institute, said southern localities should beef up surveillance of viral pneumonia and send samples from suspected cases to the institute for testing to stem possible outbreaks.
People and poultry coming from areas with bird flu outbreaks in Cambodia should be monitored, he said.
Dr Sylvie Briand, director of epidemic and pandemic preparedness and prevention at WHO, described the bird flu situation as "worrying" due to the recent increase in cases among birds and mammals, and his agency is reviewing its global risk assessment in light of the recent developments, Reuters reported.
WHO last assessed the risk to humans from avian flu as low earlier this month.
Vietnam recorded its first human case of A/H5N1 avian flu in eight years in October 2022, a five-year-old girl in the northern Phu Tho Province who had consumed meat from sick chickens and ducks a week before falling sick. She recovered after treatment.
A new strain of H5N1, clade 2.3.4.4b, emerged in 2020 and has been causing record numbers of deaths among wild birds and domestic poultry in recent months, and has also infected mammals, Reuters reported.
But unlike earlier outbreaks of H5N1, which has been around for more than two decades, this subtype is not causing significant illness in people, it said.