An estimated 9120 barrels of oil have spilled from TC Energy Corp's Keystone crude pipeline in North Dakota, state authorities said on Thursday, a major leak at a time of increased regulatory scrutiny of oil pipeline expansions.

The cause of the rupture has not yet been disclosed. But the initial estimate makes it one of the biggest onshore crude spills in the past decade and the largest for Keystone, according to US Pipeline Hazardous Materials and Safety Administration (PHMSA) data back to 2010.