BILLINGS, Mont. (AP) -- Government officials plan to haze a large herd of migrating bison back into Yellowstone National Park this week -- an annual event that is again drawing opposition from wildlife advocates and American Indian groups.

Montana state veterinarian Marty Zaluski says an estimated 400 bison are outside the park in the West Yellowstone area. Government workers could start driving the animals back into Yellowstone using a helicopter as early as Wednesday.

Hundreds of bison leave the park annually during winter to graze at lower elevations.

A state-federal agreement calls for the animals to be back in Yellowstone by May 15 to protect cattle from brucellosis. That's a disease carried by many bison.

The Montana-Wyoming Tribal Leaders Council says the bison should be allowed to return to the park on their own.

Copyright 2012 The AP.