UPDATE 2-Pemex says oil exports not hit by pipeline attack

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MEXICO CITY, Sept 10 (Reuters) - Mexico's oil output and exports were not hit by attacks on pipelines on Monday, but oil monopoly Pemex said the blasts cut off a quarter of its natural gas flow and would cost it "hundreds of millions of dollars."

Mexico's six oil refineries were operating as normal and there should be no impact on exports of crude oil, natural gas, gasoline or other fuel products, the company said.

However, Jesus Reyes Heroles, chief executive of state-owned Pemex, said it could take four or five days from Tuesday to get domestic natural gas supplies back to normal after Monday's pre-dawn sabotage attacks on half a dozen pipelines.

He estimated the pipeline damage would cut off around 25 percent of Mexico's natural gas supply for 24 to 36 hours.

"It's a major amount. We're talking about hundreds of millions of dollars," Reyes Heroles told Mexican radio.

Six attacks burst several major gas pipelines and one 30-inch crude duct, just weeks after a left-wing guerrilla group bombed other parts of Mexico's oil infrastructure.

Pemex said the explosions would slow the flow of crude to Mexican refineries, but said ample inventory meant there should be no major blow to refining and no impact on its output of gasoline and other fuels.

"We do not estimate any problem with gasoline," Reyes Heroles said.

Although it relies on imports of natural gas and gasoline to top up shortfalls in domestic output, Mexico also exports a small amount of natural gas and gasoline along with other refined fuels like diesel and jet fuel.

Reyes Heroles said the fact natural gas can't be easily stored made the situation for the domestic natural gas market more difficult. He said Pemex hoped to persuade large industrial consumers to reduce their intake slightly to avoid having to cut off supplies to smaller businesses.

Mexican glass maker Vitro (VITROA.MX) shut down five factories due to disrupted gas supplies.

Mexico is the world's No. 5 producer of crude oil and a top supplier to the United States.

Pemex is a major user of natural gas itself, but any gas it uses to run production wells is normally pumped up from the same field it is drilling.

"Production is not being affected. What is being affected is the transportation of natural gas," the company said.

In July, a shadowy leftist guerrilla group claimed responsibility for bombing energy pipelines, cutting gas supplies to thousands of businesses.

Some local media reported on Monday that the same leftist group left a message with the unexploded device -- but local government and police officials denied any note was left. "It's just a rumor," one official said.