FARGO (AP) - Environmental opponents of a proposed coal-fired power plant in northeastern South Dakota say regulators in Minnesota and North Dakota should reopen reviews after two utilities abandoned the project.

Rapidly rising costs for building coal-fired plants mean the Big Stone II plant's $1.6 billion cost estimate is too low, and understates the true costs that more than 1 million electricity customers would have to pay, said analyst David Schlissel with Synapse Energy Economics.

Schlissel was hired by Dakota Resource Council, a Dickinson-based environmental and landowner group.

Utility regulators in North Dakota and Minnesota said they want to hear from all parties before deciding how to proceed in their reviews of the project.

Great River Energy of Elk River, Minn., and the Southern Minnesota Municipal Power Agency recently said they were withdrawing as owners in the Big Stone project, which would serve electricity customers in Minnesota, North Dakota, South Dakota and two other states.

Some of the reasons cited were a drop in electric demand, higher costs and uncertainty over changes in environmental requirements and technology.

Dan Sharp with Bismarck-based Montana-Dakota Utilities Co., a spokesman for the Big Stone II utility partnership, said that a smaller version of the plant was now being planned.

"Our position is the plant is still needed," he said. "It's still the least costly alternative, the least costly option for the partner utilities and their customers."

The Minnesota Public Utilities Commission must decide whether to approve new transmission lines to deliver electricity from the plant, near Milbank, S.D., to Minnesota.

A hearing was slated for Oct. 4, but the Big Stone II partners have asked for a one-week delay. Burl Haar, executive director of the Minnesota PUC, said the delay likely would be granted.

The North Dakota Public Service Commission is weighing a request for an early determination that the Big Stone II plant would be financially prudent. Commissioners heard testimony based on a 630-megawatt plant.

"That's not the plant that's apparently out there now," Commissioner Tony Clark said. "I would need a whole lot more information. We're not going to rush through a decision this big. Obviously, the dynamics have changed a lot."