Two international organizations have stressed the high possibility of human-to-human transmission of the H7N9 virus, which saw the biggest outbreak in China over the past winter since first being reported in China in 2013.

The organizations urged the Chinese government to take more precautionary measures to prevent a possible bird flu epidemic.

"Experts agree that it is not a question of if, but when the virus will adapt in ways that facilitate efficient, sustained human-to-human transmission," according to a joint statement on Thursday from the World Health Organization and the United Nations Food and Agriculture Organization.

There was no evidence of sustained human transmission of the virus, but H7N9 has continued to change, the statement said.

Researchers recently identified two separate genetic lineages of H7N9, adding complications to the making of candidate virus vaccines, the first step in developing vaccines for birds and humans, it said.

The University of Hong Kong said on Wednesday that its researchers had identified a mutation in H7N9 avian flu that can enhance its ability to infect humans.

In February, the Chinese Center for Disease Control and Prevention said H7N9 had mutated to a new strain in South China, which did not affect the virus's ability to spread between humans but made it more dangerous among poultry.

China reported 352 human cases of H7N9 in the first two months of the year, resulting in 140 deaths, according to the National Health and Family Planning Commission.

"We have a short window of time — before the next seasonal outbreak in winter — to get ahead of the virus and prevent a SARS-type event," the joint WHO-UNFAO statement said, adding that during this short window, policymakers need to have immediate intervention steps ready to control the virus.

"This may involve a mix of market closures and poultry vaccinations as a requirement for market access," it said, adding that over 90 percent of reported cases involved individuals in direct contact with live poultry or a market selling live poultry.

Once a problem is identified, the intervention steps need to be fully implemented before the next seasonal peak, which is during the Chinese New Year holiday, it said.

Chinese authorities have taken measures since winter, such as closing live poultry markets, to control and prevent the virus.

Live poultry markets in Changsha, the capital of Hunan province, were suspended for three weeks beginning March 17, according to the city's health commission.

In Anhui province, most cities and counties have closed live poultry markets since February, local media reported.

The WHO-UNFAO statement said China needs to intensify enforcement of the closure of live poultry markets to eliminate the threat.

Sporadic cases of bird flu may last until late April, according to the China CDC. Currently, there is no vaccine for the virus, but several Chinese companies have received approval from the China Food and Drug Administration for clinical trials of vaccines.

Premier Li Keqiang said in February that China will promote business models that feature large-scale farming, centralized slaughtering and cold-chain transportation, including the keeping and selling of poultry in chilled conditions, to prevent similar poultry-related disease outbreaks.