Exxon Mobil is called on to make safety improvements to a pipeline that ruptured and spilled crude oil into the Yellowstone River, a U.S. regulator said. A leak on the Silvertip pipeline dumped around 1,000 barrels of oil into the Yellowstone River near Billings, Mont., late Friday. Exxon Mobil said it was investigating the cause of the accident. Hundreds of people were on the ground working on abatement issues but the company said flood waters were hampering some of the effort. The U.S. Department of Transportation\'s Pipeline and Hazardous Materials Safety Administration issued a corrective action order to Exxon Mobil calling for safety improvements on the Silvertip pipeline. \"It is our responsibility to ensure pipelines are safely delivering energy to U.S. households and businesses and when companies are not living up to our safety standards, we will take action,\" said U.S. Transportation Secretary Ray LaHood in a statement. Exxon under the terms of the corrective action is required to rebury the pipeline to protect it from external damage and submit a restart plan to regulators before resuming services. Exxon said it had thousands of feet of containment boom deployed along the river and was actively monitoring the plume.