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.