RENO, Nev. (AP) _ The Shoshone tribe stepped into an 18-year-old sovereignty conflict between two sisters and the federal government by releasing 50 head of cattle onto public land.

The federal Bureau of Land Management and Shoshone sisters Mary and Carrie Dann have been locked in litigation since 1974 over the BLM's allegations the Danns are overgrazing public land around their remote central Nevada ranch.

Raymond Yowell, chairman of the Western Shoshone National Council, said Thursday the situation has changed.

''The Western Shoshone Nation has nationalized the Dann sisters' livestock,'' Yowell said. ''It's no longer to the Danns but to the Shoshone nation the United States must deal with, on a nation to nation basis.''

The tribe and the federal government have had a decades-long dispute over some 12 million acres.

The Shoshone insist their nation is sovereign under the 1863 Treaty of Ruby Valley and that it and its members are exempt from BLM grazing fees.

The treaty gave white settlers the right to cross Shoshone land, but settlers began homesteading. In 1977, the Indian Claims Commission tried to pay the tribe $26 million in compensation for the land but the Shoshone have refused to touch the money.

BLM officials said no immediate action would be taken on Thursday's release of cattle.

''We don't have anyone in that neighborhood, so at the moment life goes on as normal,'' said Bob Stewart, BLM spokesman in Reno. ''Probably within the next few days we'll see what's going on and take the appropriate steps.''

The sisters, first charged in 1974 with overgrazing some 5,000 acres of BLM land surrounding their 800-acre spread, say the land is ancestral and that they can graze as many animals as they like without the United States' permission.

U.S. District Judge Bruce R. Thompson initially ruled in favor of the sisters nearly two decades ago. The case then went to the 9th U.S. Circuit Court of Appeals three times and to the U.S. Supreme Court. It was sent back to Reno, where Thompson last June dismissed the sisters' final appeal.

There are about 15,000 Shoshone in Nevada, including 5,000 on reservations.