Supplies of gasoline tightened further in parts of the United States on Tuesday as the shutdown of the nation’s biggest fuel pipeline by hackers entered its fifth day, raising concern about price spikes at the pumps ahead of the summer driving season.

The Colonial Pipeline said it was working toward a substantial restart of operations by the end of this week after a cyberattack forced it to cease operations on Friday, choking off nearly half of the East Coast's fuel supply and underscoring the vulnerability of U.S. energy infrastructure to hackers.

Colonial's website was down early on Tuesday, which Colonial said was a "temporary service disruption" unrelated to the ransomware. 

The FBI has accused a shadowy criminal gang called DarkSide of the ransomware attack. DarkSide is believed to be based in Russia or Eastern Europe and avoids targeting computers that use languages from former Soviet republics, cyber experts say.

Russia's embassy in the United States on Tuesday rejected speculation that Moscow was responsible for the attack. 

President Joe Biden on Monday said there was no evidence thus far that Russia's government was involved, but said there was evidence that the culprits' ransomware was in Russia. 

Ransomware is a type of malware designed to lock computers by encrypting data and demanding payment to regain access.

A statement issued in DarkSide's name on Monday said: "Our goal is to make money, and not creating problems for society."

It is unknown how much money the hackers are seeking, and Colonial has not commented on whether it would pay. 

The top U.S. energy regulator issued a call on Monday for mandatory cybersecurity standards for pipeline operators. "Encouraging pipelines to voluntarily adopt best practices is an inadequate response," said Federal Energy Regulatory Commission Chair Richard Glick.

Fuel supply disruption has driven gasoline prices at the pump to a three-year high and demand has spiked in some areas served by the pipeline as motorists fill their tanks.

National average pump prices are approaching their highest levels since May 2014, the American Automobile Association said. The association warned against hoarding, saying that thins supplies further. 

Florida resident Katina Willey told Reuters on Monday she went to five gas stations before she found one that had fuel available. "There were lines at three of the five stations I tried," she said. Other motorists said they were also seeking to fill up for fear the situation could worsen.

If the disruption stretches on, fuel suppliers could ship by trucks and rail instead to make up for some of the shortfalls. The Department of Transportation on Sunday lifted driver restrictions on fuel haulers in 17 states affected by the shutdown

Citgo Petroleum Corp cut production at its 418,000 barrel-per-day (bpd) Lake Charles, Louisiana, refinery and Total reduced gasoline production at its 225,500 barrel-per-day (bpd) Port Arthur, Texas due to the pipeline outage, sources told Reuters on Monday. 

U.S. Gulf Coast refiners, including Marathon Petroleum, Valero Energy, Phillips 66 and PBF Energy, booked at least four tankers to store fuel off the Gulf Coast. 

(Reporting by Laura Sanicola, Stephanie Kelly and Devika Krishna Kumar; Editing by Richard Valdmanis, Peter Cooney and Barbara Lewis) ((richard.valdmanis@thomsonreuters.com; +1 617 312 6022; Reuters Messaging: richard.valdmanis.thomsonreuters.com@reuters.net))