The company planning to take over essential air service from Big Sky Airlines says it doesn't know when it can start flying the Montana routes because it doesn't have enough aircraft to serve the seven towns.

"We do not have a timeline at all right now," said Great Lakes Aviation spokeswoman Monica Taylor. "What our holdup is, and what it has been for the past couple of months, is locating aircraft to serve the Montana communities."

The U.S. Department of Transportation, which granted Great Lakes a no-bid contract to take over service from Big Sky Airlines, said it is urging the company to move as quickly as possible. The essential air service is a federal subsidy to guarantee air routes in rural areas.

U.S. Rep. Denny Rehberg has been critical of the situation.

"There's a reason they call it essential air service," Rehberg said last week. "The Department of Transportation and Great Lakes Aviation need to live up to their commitments and find a way to keep this disruption from happening."

Rehberg said a temporary shutdown of the routes will prove costly to local residents, businesses and airports.

He said the situation is "unacceptable" and represents a failure of the DOT and Great Lakes to better coordinate.

The Cheyenne, Wyo.-based carrier earlier warned the transitions would not be easy. But Taylor said it is premature to speculate when service will resume after Big Sky, which is going out of business, terminates flights this month.

"As long as we can secure the aircraft, we are 100 percent certain we will be providing service to these cities," Taylor said.

The routes serve Glasgow, Glendive, Havre, Lewistown, Miles City, Sidney and Wolf Point, the Department of Transportation said.

"We are urging the carrier to begin service as soon as possible in order to minimize any hiatus in service," said DOT spokesman Bill Mosley.

Mosley said the federal contract, worth $8.4 million a year, given to Great Lakes has no specific dates in it. He said Great Lakes was the only company to bid on the air routes, and won't get paid until it starts flying.

Mosley said it is uncommon for service to lapse in essential air service areas. He said part of the problem was Big Sky's abrupt announcement it planned to sell its assets and essentially go out of business.

The only other essential air service town in Montana is West Yellowstone, which is served by SkyWest Airlines, Mosley said.