DALLAS (AP) — A major gasoline pipeline may be able to resume shipping fuel from the Houston area by Sunday, its operator said, potentially mitigating gasoline shortages across the southern United States since Hurricane Harvey forced Gulf Coast refineries to close. But filling tanks was already becoming difficult Thursday in central Texas, where some stations were out of gas and pump costs have risen steeply.

One Chevron station in downtown Dallas that sold regular gas for $2.29 a gallon just before the storm was charging $2.99 on Thursday. Others charged well over $3, and one downtown Shell station charged $3.97 for a regular gallon of gas. At three gas stations in north Dallas, yellow bags or caution tape was wrapped around pumps just after noon.

The supply crunch prompted QuikTrip, one of the nation's largest convenience store chains, to temporarily halt gasoline sales at about half its 135 stores in the Dallas-Fort Worth area. Instead, gasoline deliveries are going to designated stores across all parts of the metro area, QuikTrip spokesman Mike Thornbrugh said.

"Supply is way, way off," Thornbrugh said Thursday.

The Texas attorney general's office said anyone seeing gas prices of $4 or higher should take pictures and report the stations as price-gouging.

Motorists were lining up: One 7-11 had 11 cars waiting. At the "Happy's Market" Chevron, the queue stretched for a block and a half, and an Interstate 75 off-ramp was jammed with cars waiting at a Shell station. And while a dozen people lined up hoping to get gas from the single available pump at a Valero station, a driver filled a 50-gallon container in the back of his pickup.

If people start hoarding gas, "that's going to make the problem worse, and prices shoot higher and the event will last longer, with more disruption and shortages," said Patrick DeHaan, an analyst with GasBuddy.com.

His advice: "Try to have a sense of calm."

Huge challenges remain for the nation's system of getting gasoline to the pumps of service stations, since Hurricane Harvey forced the shutdown of at least eight Texas refineries, according to the American Automobile Association.

Nearly one-third of the nation's refining capacity is along the Gulf Coast from Corpus Christi, Texas, to the Lake Charles, Louisiana area, and about one-quarter of the Gulf Coast's oil refining capacity was taken offline, according to the Oil Price Information Service.

Nationwide, pump prices have surged — the average for a gallon of regular gasoline rose from about $2.35 a week ago to $2.45 now, AAA reported. The price spike is more dramatic in Texas, of course, and in states such as Georgia, where the average cost per gallon of regular gas has climbed from $2.22 a week ago to $2.39 now.

The Colonial Pipeline provides nearly 40 percent of the South's gasoline. It runs underground and is now under water in many parts of Texas, where inspections are needed before it can be fully operational again, Colonial spokesman Steve Baker said Thursday.

The Georgia-based company remains able to operate its pipeline from Louisiana to states east and northeast of there, though deliveries will be "intermittent," the company said.

The Colonial Pipeline, a crucial artery in the nation's fuel supply network, runs from the Houston area to New York harbor and includes more than 5,500 miles of pipeline, most of it underground. It closed in September 2016 after a leak and gas spill in Alabama, leaving empty pumps and higher prices in Alabama, Georgia, Tennessee and the Carolinas.

Oklahoma-based QuikTrip diverted gasoline deliveries to its store in a similar way last year in metro Atlanta, where it has about 133 stores, after the Alabama pipeline spill.

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Martin reported from Atlanta and Lauer from Dallas.