BISMARCK, N.D. (AP) -- 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.

South Bow is still investigating the cause of the spill Tuesday along its pipeline near Fort Ransom, North Dakota, about 60 miles (97 kilometers) southwest of Fargo, the company said.

The spill released an estimated 3,500 barrels, or 147,000 gallons of oil, onto farmland. The company said 700 barrels, or 29,400 gallons, have been recovered so far. More than 200 workers are on-site as part of the cleanup and investigation.

South Bow has not set a timeline for restarting the 2,689-mile (4,327 kilometers) pipeline, which stretches from Alberta, Canada, to refineries in Illinois, Oklahoma and Texas. The company said it "will only resume service with regulator approvals."

South Bow is working with the federal Pipeline and Hazardous Materials Safety Administration and the state Department of Environmental Quality.

Continuous monitoring of air quality hasn't indicated any adverse health or public concerns, South Bow said.

The site remains busy, said Myron Hammer, a nearby landowner who farms the land affected by the spill. Workers have been bringing in mats to the field so equipment can access the site, and lots of equipment is being assembled, he said.

The area has traffic checkpoints, and workers have been hauling gravel to maintain the roads, Hammer said.

There is a cluster of homes in the area, and residents include retirees and people who work in nearby towns, he said. But the spill site is not in a heavily populated area, Hammer said.