The Pipeline and Hazardous Materials Safety Administration gave Exxon Mobile Pipeline Co. until April 7 to submit a work plan. Monday was the previous deadline.
About 5,000 barrels of a diluted form of heavy Canadian crude oil spilled from a 22-foot rupture in the pipeline in Mayflower, Ark., in March.
Richard Kuprewicz, a pipeline safety consultant working for Arkansas regulators, told Inside Climate News extensions were fine provided Exxon was making real commitments to prevent future incidents.
"What's unbelievable is the lack of details to show/demonstrate meaningful progress," he said in an interview published Tuesday.
Exxon said it wouldn't restart the pipeline until it was safe to do so.
U.S. Rep. Tim Griffin, R-Ark., said in November a letter to the PHMSA he was opposed to restarting the pipeline. He wants Exxon to relocate a 13-mile section near a watershed to ensure the safety of the drinking water.
A manufacturing defect from the 1940s was likely to blame for the pipeline's crack, the PHMSA said.