There was no clear end in sight on Monday to the outage stemming from a cyberattack that forced the largest refined products pipeline in the United States offline late last week.

Colonial Pipeline

Colonial Pipeline Co., a major supplier of gasoline, diesel and jet fuel to the eastern United States, said in a statement over the weekend that it halted operations across a pipeline system that spans more than 5,500 miles on Friday after learning it was the victim of a ransomware attack. Hackers infiltrated some of its information-technology (IT) systems.

In a statement Monday afternoon, the company said it was still working to address the scope of the security threat and restart a pipeline system that transports nearly half of the East Coast’s fuel supply from the Gulf Coast. Colonial has not said...