The federal government is warning the Cherokee Nation that its expulsion of about 2,800 descendants of slaves once owned by its members violates an 1866 treaty and is unconstitutional.

The assistant secretary for Indian Affairs called on the tribe Tuesday to restore the descendants' membership, including voting rights and benefits such as medical care and food stipends.

The Bureau of Indian Affairs says unless the tribe re-admits the descendants, a Sept. 24 special election to elect a principal chief won't be valid. The Cherokee Supreme Court ordered the special election after several counts and recounts in a June election ended with each candidate declared the winner twice.

The tribe voted in 2007 to expel the descendants but didn't take action until the U.S. Supreme Court upheld the vote last month.