Workers have recovered thousands of gallons of crude oil from an underground pipeline spill on North Dakota farmland, the owner of the line said Thursday, but it remains unclear when oil will again start flowing to refineries.

A portion of the Keystone Pipeline ruptured near Fort Ransom, North Dakota, around 7:44 a.m., according to local reports.

Officials do not know how much oil leaked from the Keystone Pipeline after an employee heard a loud noise Tuesday morning.