By Laura Zuckerman

(Reuters) - The southwestern U.S. population of endangered Mexican gray wolves declined by 12 percent last year after five years of steady growth, leading wildlife advocates to suggest that illegal killings of the beleaguered predators may be to blame.

Wildlife managers said Thursday the drop - from 110 wolves in 2014 to 97 last year - was unexpected and disturbing but that federal and state governments should stick with their decades-long recovery efforts for the animal.

The tally did not include an estimated 20 more Mexican wolves roaming south of the U.S. border.

“The lower number of Mexican wolves that were counted is a concern, but not a signal that the program is unsuccessful,” Jim de Vos, assistant director of wildlife management for the Arizona Game and Fish Department.

The U.S. population of the Mexican wolf, the rarest subspecies of gray wolves in North America, had been gradually expanding over the previous five years, averaging 11 percent annual growth in southern New Mexico and southeastern Arizona, said John Bradley, spokesman for the U.S. Fish and Wildlife Service.

Factors that led to last year's decline include the deaths of 13 wolves, nine of them females, and a sharp decrease in the number of wolf pups that survived through December, government wildlife managers said.

Mexican wolves were believed to be all but extinct in the United States and Mexico when the last five were caught alive in Mexico between 1977 and 1980 and used for a captive breeding program that saw 11 animals released to the wild in the two southwestern U.S. states in 1998.

Like their larger counterparts in the Northern Rockies and the U.S. Midwest, Mexican gray wolves draw the ire of ranchers for preying on cattle and are a big game prized by hunters.

But conservationists have hailed the comeback of wolves in states like Idaho, Montana and Michigan as a triumph for keystone predators that were once hunted, trapped and poisoned to near extinction before coming under U.S. Endangered Species Act protections in the 1970s.

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Wildlife advocates decried the 2015 decline in Mexican wolves and pointed to illegal killings as a probable factor. Last year's deaths, including two females killed accidentally after being captured by government biologists for re-collaring, are under investigation, the Fish and Wildlife Service reported.

Michael Robinson of the Center for Biological Diversity faulted the agency for loaning transmitters to ranchers to better track radio-collared wolves in the vicinity of their livestock.

(Reporting by Laura Zuckerman from Salmon, Idaho; Editing by Steve Gorman and Michael Perry)