U.S. scientists are estimating that between 5.7 million and 6.7 million bats in Canada and the United States have succumbed to white-nose syndrome, a fungus spreading in eastern North America.

The mortality figures were released Tuesday by the U.S. Fish and Wildlife Service.

"White-nose syndrome has spread quickly through bat populations in eastern North America, and has caused significant mortality in many colonies," said  Dr. Jeremy Coleman, co-ordinator of the U.S white-nose syndrome program. "Many bats were lost before we were able to establish pre-white-nose syndrome population estimates."

U.S. Fish and Wildlife Service director Dan Ashe said the "startling " figures illustrate the severity of the threat facing the animals.

"Bats provide tremendous value to the U.S. economy as natural pest control for American farms and forests every year, while playing an essential role in helping to control insects that can spread disease to people," Ashe said.

White-nose syndrome was first detected in a cave west of Albany, N.Y., in 2006. It has since spread to 16 states, along with Ontario, Quebec, New Brunswick and Nova Scotia. Scientists expect the disease will continue to spread to other regions.

Bats with the syndrome exhibit unusual behavior during cold winter months, including flying outside during the day and clustering near the entrances of caves and mines where they hibernate. The disease is named for the white patches that appear on the muzzles and other body parts of hibernating bats.

Advocates says action is needed to protect bats.

"Today's new mortality estimates are a wake-up call that we need to do more, and fast," said Mollie Matteson with the Center for Biological Diversity.

With from The Associated Press