The new H7N9 bird flu virus can be transmitted between mammals not only via direct contact but also in airborne droplets, and may be capable of spreading from person to person, Chinese and American researchers have found.
A study published in the journal Science and presented at a briefing in Hong Kong on Friday found that three ferrets - an animal often used for research on flu - that were in the same cage as ferrets infected with H7N9 had contracted the disease.
One of three ferrets kept in separate cages nearby also became infected, through airborne exposure.
The World Health Organization (WHO) has previously said it has no evidence of "sustained human to human transmission" of the virus, which has killed 36 people in China.
"The findings suggest that the possibility of this virus evolving further to form the basis of a future pandemic threat cannot be excluded," said the research team, led by bird flu expert and microbiologist Yi Guan.
The virus can also infect pigs, but could not be transmitted from pig to pig or from pigs to other animals, the study showed, although the team urged authorities to maintain surveillance to check whether the virus was mutating.
The WHO said the findings were useful but warned that people "have to be very careful about what's going on on the ground".
"Studies like that are really helpful for increasing general knowledge and it's really helpful to know that, under lab conditions, this thing could transfer from person to person," WHO chief spokesman Gregory Hartl told Reuters.
"We've already seen maybe a few limited instances of human to human transmission within close family range, within close contacts, so this is another piece of the puzzle," he said.
The findings come just days after the WHO said the H7N9 virus appeared to have been brought under control in China thanks to restrictions at bird markets.
H7N9 has relatively mild clinical signs in ferrets, according to the study. All the animals infected with the virus in the experiments presented symptoms for no more than seven days and all recovered from the disease.
The researchers said that all cases where humans had died or become extremely ill had involved additional factors.
The team also found that some infected animals did not develop fever or other clinical signs, suggesting that asymptomatic infections among humans may also be possible.
"The potential public health implication of this ... is that a person infected by H7N9 avian influenza virus who does not show symptoms could nevertheless spread the virus to others," the researchers wrote in their study.
United Nations experts said this week the bird flu outbreak in China had caused some $6.5 billion in losses to the economy.
The H7N9 virus is known to have infected 131 people in mainland China and one in Taiwan since February, but no new cases have been detected since early May.