Pacific walruses need additional protection from the threat of climate warming but cannot be added to the threatened or endangered list because other species are a higher priority, the US Fish and Wildlife Service said.

Walruses will be added to the "warranted but precluded" list, said agency spokesman Bruce Wood, a designation under the Endangered Species Act that allows delays in listing if the agency is making progress listing other species and does not have resources to make a decision on others.

"The threats to the walrus are very real, as evidenced by this 'warranted' finding," said Geoff Haskett, the service's Alaska region director, in a statement.

"But its greater population numbers and ability to adapt to land-based haul-outs make its immediate situation less dire than those facing other species such as the polar bear."

He said co-operation with Alaska Native groups, the state and other partners could lessen the long-term impact of climate change for the walrus and help it avoid an endangered listing.

The decision was condemned by the Centre for Biological Diversity, which in 2008 petitioned to list walruses as threatened or endangered, citing threats to walruses' sea ice habitat.

Centre spokeswoman Shaye Wolf said the warranted but precluded designation is a black hole for endangered species. Some have been so designated for more than 20 years.

"This decision acknowledges the walrus is facing extinction due to climate change but the Obama administration is withholding the protections that could help the walrus survive," Ms Wolf said.

Scientists at the National Snow and Ice Data Centre have tracked a steady decline in sea ice in recent decades. Climate models have projected that summer sea ice could disappear by 2030.

Alaska's walrus population spends virtually the entire winter in the Bering Sea on the edge of sea ice that forms every year. In spring, as temperatures warm, ice melts and the edge of the sea ice moves north.

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