The U.S. Centers for Disease Control and Prevention (CDC) said on Thursday it was monitoring a new strain of bird flu and has started work on a vaccine just in case it is needed.
So far, the strain known as avian influenza A (H7N9) is only in China and has not yet been found to be capable of being transmitted from person to person.
The strain has killed five people, and global health officials are debating whether to start mass-producing a vaccine.
The CDC said on its website it was following the situation closely and coordinating with domestic and international partners.
CDC spokesman Tom Skinner said the agency was reviewing genetic sequence information on the strain and has begun working on a vaccine using synthetic genes.
The infections in China mark the first time humans have been afflicted by this new strain of bird flu, which causes severe respiratory illness.
The CDC said the steps it was taking are "routine preparedness measures" that apply whenever a new flu virus is detected in people.