Hong Kong (CNN) -- China announced another case of a rare strain of bird flu Monday, taking the total number of H7N9 infections to 21, the state-run Xinhua news agency said.
The new case was confirmed in the country's eastern Anhui province, Xinhua said.
Six people have died so far, four in Shanghai and two in Zhejiang, in eastern China, Xinhua said.
Chinese authorities have killed more than 20,000 birds from a live-poultry trading zone in Shanghai in an effort to deal with the issue, while a number of other cities across the country have also announced trading suspensions.
In a joint press conference with China's health ministry in Beijing early Monday, the WHO reiterated that the source of the current outbreak has yet to be pinpointed but that they have not seen any cases of the virus passing between humans.
"The recent reports from China are the first cases of human infection with H7N9 viruses. Although we do not yet know the source of infection, at this time there is no evidence of sustained human-to-human transmission," said the WHO's Michael O'Leary.
"We know at this point that, in the human cases we know, the disease is very serious and a large percentage of cases have died -- or a substantial percentage have died and others are critically ill.
"Some of the confirmed cases had contact with animals or with environments in which animals were located. The virus has been found in a pigeon in a market in Shanghai. These events gave possibility of animal-to-human transmission, for which investigations continue."
Meanwhile, Chinese officials say they have started work on a vaccine for the H7N9 virus.
However, it may take up to eight months before the vaccine can be brought to market due to complications in the development and manufacturing process, said Liang Wannian, director of the H7N9 influenza prevention and control office under the National Health and Family Planning Commission (NHFPC), according to Xinhua.
Liang said the decision to put the vaccine into production will depend on whether the virus can mutate to become human-to-human transmissible.
"We are taking a series of measures to prevent and control infections. Infection monitoring efforts are under way in over 500 hospitals and 400 labs nationwide. We have confidence in keeping the disease under control," he said.
Ke Feng in Beijing contributed to this report.