GREAT FALLS, Mont. (AP) — Montana's U.S. senators have joined six others in asking the Defense Department to hold off on an environmental assessment on Minuteman III intercontinental ballistic missile silos.

The Great Falls Tribune reports the study is required before the Air Force can make any changes to the structure of the ICBM force.

The 450 ICBMs are in Montana, North Dakota and Wyoming.

Last week, President Barack Obama signed legislation allowing the Defense Department to begin the study.

The legislation withholds half the funds until the department gives Congress options for the nuclear force structure under the New START treaty with Russia.

Sens. Jon Tester and Max Baucus are asking the department to await a decision by Congress on funding for 2014 before beginning the study.

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