Colonial Pipeline has begun to restart the United States’ largest pipeline network, six days after a ransomware attack prompted it to shut the line, triggering fuel shortages and panic buying in the southeastern United States.

It will take several days for the fuel delivery supply chain to return to normal, Colonial said on Wednesday, even as people in southeastern states scramble to fill their tanks as stations ran out of petrol.

Observers reported fistfights erupting over fuel supplies in North Carolina and other places.

Colonial halted 2.5 million barrels per day of shipments of petrol, diesel and jet fuel last Friday after the most disruptive cyberattack ever on US energy infrastructure.

Sources familiar with Colonial’s response said the company does not plan to pay the ransom demanded by hackers who encrypted data on the pipeline, which stretches 8850 kilometres from Gulf Coast oil refineries to consumers in mid-Atlantic and southeast states.

The supply crunch sparked panic buying in the US southeast, bringing long lines and high prices at service stations before the Memorial Day holiday weekend at the end of May, the traditional start of the peak summer driving season.

Nearly 60 per cent of service stations in metro Atlanta were without petrol, tracking firm GasBuddy said. Its survey showed 65 per cent of stations in North Carolina and 43 per cent in Georgia and South Carolina without fuel. Virginia also reported high outages.

US Transportation Secretary Pete Buttigieg said the administration was addressing fuel shortages and helping restore Colonial operations.

“Our top priority right now is getting the fuel to the communities that need it, and we will continue doing everything that we can to meet that goal in the coming days,” Buttigieg told reporters at the White House.

Privately owned Colonial Pipeline opened portions of the line manually in Georgia, Maryland, New Jersey and the Carolinas. It also accepted two million barrels of fuel to begin efforts to “substantially” restore operations by week’s end, the company has said.

The average national gasoline price rose to above $US3 ($A3.9) a gallon, the highest since October 2014, the American Automobile Association said.

The FBI has accused a shadowy criminal gang called DarkSide of the ransomware attack. The group, believed to be based in Russia or Eastern Europe, has not directly taken credit for the Colonial hack, but on Wednesday it claimed to have breached systems at three other companies, including an Illinois tech firm.

Russia’s embassy in the United States rejected speculation that Moscow was behind the attack. On Monday, Biden said there was no evidence so far that Russia was responsible.

It was not known how much money the hackers are seeking.

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