The United States is pressing Mexico, the top importer of U.S. chickens and turkeys, to relax restrictions imposed on poultry shipments because of an outbreak of a strain of avian flu deadly to birds, an Agriculture Department spokeswoman confirmed on Friday.

Negotiations are playing out as the number of U.S. poultry flocks infected with the disease continues to rise and the cost of the U.S. government response is approaching $50 million.

Government officials are aiming to limit the economic pain the H5N2 flu causes to producers and exporters by asking Mexico to restrict imports only from counties that are home to infected flocks. The disease can kill nearly an entire infected flock of poultry within 48 hours. 

So far, Mexico has imposed trade restrictions on entire states, including top U.S. turkey producer Minnesota, when infections have been detected.

Since the beginning of the year, H5N2 has been found in birds in 12 states stretching from Oregon to Arkansas. Mexico has limited poultry imports from states with infections in commercial operations.

The most serious trade reaction triggered by the flu outbreak so far has come from China and South Korea, which have cut off all imports of U.S. poultry and poultry products in the $5.7 billion export market.

Wild waterfowl are thought to be spreading the bird flu, and "more detections are expected during the spring migration," the USDA said in a notice announcing new cases on Friday.

Six more flocks of turkeys in Minnesota, Wisconsin and South Dakota, comprising almost 350,000 birds, were confirmed to be infected with the H5N2 flu, according to the USDA. Birds in the flocks will be culled to prevent the spread of the disease and will not enter the food system, the agency said.

So far, the USDA has spent about $45 million responding the flu outbreak, including costs for testing, quarantines around infected facilities, and compensation for producers whose birds have been killed by the virus or culled to prevent the spread of the disease, an agency spokeswoman said.

No human cases of the flu have been detected.