By Ernest Scheyder

Jan 22 (Reuters) - Nearly 3 million gallons of saltwater and an unknown quantity of crude oil have leaked from a North Dakota pipeline into a creek that feeds the Missouri River, by far the largest spill of its kind in the state's history, officials said.

The leak, from a saltwater collection line owned by Summit Midstream Partners LP approximately 15 miles north of Williston, occurred sometime earlier this month and was reported to state officials on Jan. 7.

The leak does not pose a threat to drinking water supplies, the North Dakota Department of Health said in a statement released late Wednesday.

Saltwater is a byproduct of the hydraulic fracturing process. Typically it is filtered and re-injected back into the earth after oil is extracted.

The saltwater leaked into a creek that passes by Williston, considered the capital of the state's oil boom, and flows into the Missouri River. Williston's drinking water comes from the Missouri River, though the city's water department has the ability to turn off collection valves until any harmful material washes downstream.

Summit has hired environmental contractors to clean up the spill. About 2 million gallons of water have been pulled so far from one of the affected creeks, though it was not immediately clear if that amount was all saltwater or normal water flow.

Remediation will be difficult given that much of the affected area is covered by ice.

A Summit spokesman was not immediately available to comment.

The state's Department of Health said it is monitoring cleanup efforts, and the state's Department of Mineral Resources is inspecting Summit's entire pipeline network, officials said.

(Reporting by Ernest Scheyder; Editing by Leslie Adler)