A spokesperson for the Geneva-based global health agency says China is starting to test for the new strain across the country.
To date cases have been found only in four provinces in eastern China, near Shanghai.
But Gregory Hartl says Chinese authorities are starting to test for the virus through the country's surveillance network for influenza-like illnesses.
That means people sick with flu-like illnesses who seek care at sites in the network across China could be tested for H7N9.
To date China have confirmed 24 infections, with seven deaths.
It's not clear whether the clustering of cases in the four provinces means that those are the only places where infections are occurring, or whether broadening testing will turn up cases in more far-flung parts of the vast country.
Hartl said the work to find the animal reservoirs of the virus is also continuing. China's Agriculture Ministry and the U.N. Food and Agriculture Organization are collaborating, he said, and have collected more than 11,000 samples from various animal types to look for where the virus is hiding in nature.
So far H7N9 has been found in pigeons, quails and chickens.
Finding the virus in animals has been challenging because it currently doesn't sicken or kill poultry.
H7 flu viruses, like the better known H5 viruses, can come in two forms. They can be highly pathogenic, killing poultry, or can come in a low pathogenicity form. The new virus is the latter, meaning it can spread in a virtually invisible manner among poultry flocks.
However, low path H7 and H5 viruses will typically evolve to become high path viruses, if they circulate among poultry for long enough. (The designations of low path or high path relate to how the viruses act in poultry, and not how deadly — or not — these viruses are for people who become infected with them.)
Hartl says human infections continue to accumulate at a rate of two or three a day, but the WHO believes the cases are still sporadic, with "almost no evidence of human-to-human transmission of any type."
He explained he used the word "almost" because among the more than 600 contacts of cases that China is investigating, a few have shown signs of illness and need to be followed up.
At this point there may have been rare occasions of limited human-to-human spread, of the type that is occasionally seen with H5N1 bird flu, he acknowledged. Small clusters of H5N1 cases have occurred, but only among people who have had close contact with someone ill with the virus.
With H7N9, "there is absolutely no evidence of any sustained human-to-human transmission," Hartl says.
Experts trying to determine whether a flu virus is on the verge of breaking out to cause a pandemic look for easy and sustained spread from person to person, the type of transmission one sees with seasonal flu viruses.
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