The Center for Biological Diversity filed a legal petition today urging the US Fish & Wildlife Service (FWS) to restore grizzly bears to their iconic American West habitat, in line with requirements of the Endangered Species Act.

The petition identifies 110,000 square miles of potential grizzly habitat, including such places as California's Sierra Nevada and the Grand Canyon in Arizona.

These areas, the conservation group says, still contain vast regions of undeveloped habitat favored by the bears.

"We've just begun the job of recovering grizzlies. There is so much more to do to see the bears restored to more of their range in the western United States," Noah Greenwald, endangered species director for the Center for Biological Diversity, told Reuters.

Some 1,600 grizzlies currently roam Yellowstone National Park and its border states of Idaho, Montana and Wyoming. The push by the Center for Biological Diversity would see their numbers triple to 6,000 in the US West, including parts of Arizona, Colorado, New Mexico and Utah.

"Grizzly bears are one of the true icons of the American West, yet today they live in a paltry four percent of the lands where they used to roam," Greenwald added in a press release.

Since grizzly bears were listed as a threatened species under the Endangered Species Act in 1975, the Center believes it's the FWS's responsibility to see that their natural habitat is restored, since they have failed to do so thus far, so that populations may flourish again.

To date the agency has developed recovery strategies for only six populations, and for the most part they have only carried out such efforts in the first four populations, according to the Center's press release.

All of these populations are isolated, Greenwald said, especially in Yellowstone. Failure to expand their habitat could result in inbreeding and the inability to adapt to warmer climates - factors that may one day turn this threatened species into an endangered one.

"The good news is that with the safety net of the Endangered Species Act, the health of Yellowstone and Glacier area grizzly bears has improved - but it's way too early to declare victory and walk away," Greenwald added.

This is not the group's only quarrel with the FWS. The Center for Biological Diversity is also in the midst of suing the agency for allegedly disregarding protection needs for a rare Florida snail species under the Endangered Species Act.