Top U.S. fuel pipeline operator Colonial Pipeline said on Monday that a section of the system from North Carolina to Maryland had been reopened at least temporarily after all pipeline operations were halted due to a cyber attack.

"We can now report that Line 4, which runs from Greensboro, N.C., to Woodbine, Md., is operating under manual control for a limited period of time while existing inventory is available," said the company in a statement.

However, its main lines remained shut.

The U.S. government declared a state of emergency on Sunday in response to a ransomware attack that forced Colonial Pipeline to shut a critical fuel network supplying nearly half the fuel consumed along the country's East Coast.

The Federal Bureau of Investigation attributed the cyber attack to DarkSide, a group believed to be based in Russia or Eastern Europe. Its ransomware targets computers that do not use keyboards in the languages of former Soviet republics, cyber experts said.

Ransomware is a type of malware designed to lock computers by encrypting data. The hackers demand payment to let the owner regain access. It is unknown how much money the hackers are seeking, and Colonial has not commented on whether it would pay.

The episode laid bare the vulnerabilities of energy infrastructure to hackers. U.S. lawmakers responded with calls for stronger protections for critical energy infrastructure.

While the impact remains to be quantified, the pipeline shutdown will reduce fuel availability in the near term, push up prices and force refiners to cut production because they have no way to ship the gas.

If the disruption stretches on, fuel suppliers could ship by truck and rail instead. The U.S. Department of Transportation on Sunday lifted driver restrictions on fuel haulers in 17 states affected by the shutdown.

U.S. fuel importers are also booking tankers to bring gasoline from Europe. U.S. refiners including Motiva Enterprises and Total cut output at Gulf Coast plants on Monday because of a lack of transport.

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U.S. declares state of emergency over Colonial pipeline shutdown

(With input from Reuters)