The operator of the United States' largest fuel pipeline, which was shut down by a cyber attack, says it has returned to "normal operations", easing concerns but not immediately ending gas shortages affecting many eastern US states.

Late on Thursday, Colonial Pipeline said its system was back up and running but that it might take days for the supply delivery chain to return to normal. 

But on Saturday (local time), the Georgia-based company announced the full return to normal, "delivering millions of gallons per hour to the markets we serve", it said on Twitter.

Washington believes a Russia-based criminal group known as Darkside had targeted the company in a so-called ransomware attack. 

On Friday, computer servers for Darkside were taken down by unknown actors.

Security firm Recorded Future said Darkside had admitted in a web post that it lost access to certain servers, though reportedly only after receiving a $US5 million ($6.4 million) ransom payment from Colonial Pipeline.

Former US Treasury secretary Larry Summers said on CNN that US officials had likely exerted heavy pressure on the Russian leadership to shut down the hackers.

Colonial's temporary shutdown led to panic buying, with frantic motorists from Florida to Maryland lining up at petrol stations.

The resultant surge in demand sent the national average price above $3 a gallon for the first time since late 2014.

Website GasBuddy said 81 per cent of stations in Washington remained out of gas on Saturday, though the numbers in several eastern states were much better. 

Colonial Pipeline operates the largest fuel conduit system in the United States, sending gasoline and jet fuel from the Gulf Coast of Texas to the populous east coast through a pipeline network serving 50 million consumers.

AFP