[Photo] Some 1,500 grey wolves now roam Idaho, Montana and Wyoming(Image: WWF-Canon/Chris Martin Bahr)

Grey wolves in the Northern Rockies of the United States have been removed from the endangered species list, the US Department of the Interior has said.

The move follows efforts over the last 13 years to protect the animals and allow their population to grow.

There are now an estimated 1,500 grey wolves in Idaho, Montana and Wyoming.

They became a protected species in the US after they were nearly hunted to extinction. The removal of protection means they can be hunted again.

Environmental groups have said they will sue the federal government to keep the animal listed.

"The wolf population is doing great. The ESA [Endangered Species Act] worked. We've got a lot of wolves in a lot of places," Ed Bangs, wolf recovery co-ordinator for the US Fish and Wildlife Service, said before the announcement.

Last year, the population of grey wolves in the western Great Lakes area was removed from the endangered species list.