BILLINGS, Mont. (AP) — A federal judge has denied a request by Native American tribes to halt construction of the Keystone XL oil pipeline from Canada over worries about spills and damage to cultural sites.

Work started this spring on the long-stalled pipeline that would carry oil sands crude from Hardisty, Alberta to Steele City, Nebraska.

The Assiniboine and Gros Ventre tribes of the Fort Belknap Indian Community in Montana and Rosebud Sioux Tribe in South Dakota are challenging President Donald Trump’s 2019 permit for the project.

U.S. District Judge Brian Morris says he had “serious questions” about the legal claims being made by the tribe. A final ruling is pending.