"Following a review of the best available scientific information, the US Fish and Wildlife Service today proposed listing the African lion as threatened under the Endangered Species Act," the FWS said in a statement.
"The agency's analysis found that lions are in danger of extinction in the foreseeable future."
Such protection would allow US authorities to toughen enforcement and monitoring of imports and international trade, the FWS said.
Listing an animal under the Endangered Species Act can also help raise awareness and money to help conservation efforts.
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Human settlements are encroaching on lions' territory, putting livestock within easy reach of the big cats and causing conflict with people.
Even more, the animals that lions typically eat are being increasingly hunted and consumed by people.
"As a result, lions kill more livestock, which then leads to retaliatory killings by humans," said the FWS statement.
The FWS decision comes after a 2011 petition from a coalition of organisations requesting the African lion be listed as endangered.
A 90-day comment period will now begin, in which the public can weigh in on the proposed rule. The window closes January 27, 2015.