The American Medical Association is asking U.S. antitrust officials to block or modify a recently announced managed care deal.

If officials let UnitedHealth Group Inc., Minnetonka, Minn., complete the recently announced acquisition of Sierra Health Services Inc., Las Vegas, as planned, UnitedHealth will end up with 95% of the Las Vegas-Paradise metropolitan area health maintenance organization market and 78% of the Nevada HMO market, according to the AMA, Chicago.

“United’s near-monopoly in the HMO insurance market will deter competition and deny patients and employers a choice among HMO plans,” AMA Board Member Dr. J. James Rohack says in a statement about the proposed deal.

The AMA notes that federal officials recently made UnitedHealth divest some holdings when it acquired another regional managed care company, PacifiCare Health Systems Inc., Cypress, Calif., in 2005.