– U.S. agriculture officials seeking ways to control deadly bird flu have approved the use of a method to kill poultry by turning up the heat in their barns and shutting off ventilation systems.

The Agriculture Department will consider using the method, condemned by animal rights groups as cruel, if there are no other ways to kill flocks within 24 hours of flu infections being detected, according to a statement.

The agency has set a goal to cull infected flocks within a day to prevent the virus from spreading, after nearly 50 millions chickens and turkeys died from December through June in the nation's worst-ever animal disease outbreak.

More than two months have passed since the last infection was detected. However, officials are preparing for the potential return of the flu this fall because wild ducks, which are thought to carry the virus, will begin migrating.

Shutting off ventilation systems is "considered by some to be less humane than other methods" of culling flocks, the USDA said in a statement.

However, the method, known as ventilation shutdown, is a "necessary alternative" because of the need to control and eradicate the virus, according to the agency.

It takes about 30 to 40 minutes for the birds to die from heat stress during ventilation shutdown, said T.J. Myers, associate deputy director of surveillance, preparedness, and response services for the USDA's Animal and Plant Health Inspection Service. The USDA has never used the method, he said.

The government is trying to improve its response to the flu after farmers and lawmakers complained that the agency moved too slowly in killing infected flocks and disposing of them. Delays can contribute to the spread of the disease and keep farmers out of business.

Shutting down ventilation systems in poultry houses "essentially bakes the birds to death," according to the Humane Society of the United States.

The USDA said it plans to approve the use of ventilation shutdown on a case-by-case basis.