HELENA (AP) — NorthWestern Energy said Tuesday that the Federal Trade Commission approved its proposed $900 million purchase of 11 hydroelectric dams in Montana, a move that could raise electric rates about 4 percent.

The company issued a statement saying it’s a significant step, but it still must be reviewed and approved by the state Public Service Commission and the Federal Energy Regulatory Commission.

NorthWestern, the largest electric and natural gas utility in Montana, believes the dams will provide an affordable, long-term supply of electricity for customers, Lee Newspapers of Montana reported.

“We’ve passed a major milestone at the federal level,” said John Hines, vice president of supply for the company.

NorthWestern announced last September that it had reached an agreement to buy 11 hydroelectric dams and a storage dam from PPL Montana.

The dams, which are on the Missouri, Madison, Flathead and Clark Fork rivers and West Rosebud Creek, were owned by the Montana Power Co. until 1999, when PPL Montana bought them.

A witness for the Montana Consumer Counsel has said the deal could shift millions of dollars onto consumers unless more risk for the cost of the purchase is assumed by the utility.

The five-member Public Service Commission will be holding hearings on the proposal and make a decision later this year.

NorthWestern said it received a notice from the FTC on Monday that the purchase does not violate antitrust laws but had no further details on the agency’s ruling.