DULUTH — Cliffs Natural Resources said it has reached an agreement to provide taconite pellets to steel-producing giant AK Steel through 2023.

The agreement extends a previous contract that runs through 2014.

Cliffs said Tuesday the supply agreement is tied to a moving price index based on changing iron ore prices and producer price indices.

Cliffs won’t say which of its mines supplies AK mills.

Cliffs operates United and Northshore taconite operations and is co-owner of and operates Hibbing Taconite, all in Minnesota.

The Cleveland-based company also owns the Empire/Tilden operation in Michigan’s Upper Peninsula and has iron ore mines in eastern Canada and Australia.

Ohio-based AK Steel is one of the nation’s largest steel producers.