Conservationists welcomed Tuesday a US appeals court ruling that grizzly bears still need protecting, after federal authorities sought to have them taken off an endangered species list.

The Ninth Circuit Court ruled that the US Fish and Wildlife Service cannot take away Endangered Species Act protection from grizzlies in the Greater Yellowstone region of the Rocky Mountains.

Specifically it said the disappearance of whitebark pine, a crucial food source for grizzlies, potentially threatens the long-term survival "ursus horribilis," reports said.

"This case involves one of the American Wests most iconic wild animals in one of its most iconic landscapes," wrote Richard Tallman a member of the three-judge panel which returned the verdict.

"Based on the evidence of a relationship between reduced whitebark pine seed availability, increased grizzly mortality to reduced grizzly reproduction, it is logical to conclude that an overall decline in the regions whitebark pine population would have a negative effect on its grizzly bear population."

The former Seattle lawyer was cited by the Seattle Post-Intelligencer newspaper as saying: "Now that this threat has emerged, the Service cannot take a full-speed-ahead, damn the torpedoes approach to de-listing."

Mike Clark, executive director of conservation group the Greater Yellowstone Coalition, hailed the verdict.

"We appreciate the strong language of the 9th Circuit Court saying that USFWS must further study the demise of the whitebark pine and its impact upon grizzlies before it can delist the Yellowstone griz," he said in a statement.

"Secondly, we look forward to working with the feds and state officials on plans that ultimately will delist the griz when it is appropriate. But the court has clearly ruled that such a time is not yet upon us."

Grizzlies used to range widely across the Rocky Mountains and the Great Plains, but hunting drastically reduced their numbers.

Today they are found only in scattered locations, mainly national parks including Yellowstone, which covers parts of the US states of Montana, Idaho and Wyoming.

They can weigh up to 1,500 pounds (680 kilograms) and sport large shoulder humps. Despite their size, they can run up to 35 miles (55 kilometers) per hour, according to the US Fish and Wildlife Service.