MEXICO CITY, Oct 19 (Reuters) - A Mexican rebel group behind a spate of fuel pipeline bombings this year repeated a threat to carry out more attacks unless two missing activists it says the government is holding are returned.

The Marxist-inspired Popular Revolutionary Army, or EPR, said if it were not taken seriously, it would step up its campaign.

The government denies it is holding the missing activists, who the EPR says were captured in May in the volatile southern city of Oaxaca, and says it has no idea of their whereabouts.

The two men, one of whom is the brother of EPR leader Tiburcio Sanchez Cruz, had been living clandestinely for years.

"We are waiting, patiently and with caution, for clear and concrete signs and we say to them that this is the moment to take our demands seriously so that we are not obliged to step up our national harassment campaign," the EPR said in a statement posted on a Web site used by leftist rebel groups.

Quiet since the 1990s, the EPR burst back onto the scene this year, bombing fuel pipelines in July and September. The attacks disrupted supplies of crude oil to refineries and cut off natural gas supplies to hundreds of manufacturers.

The shadowy group, believed to number just a few hundred members, calls for land reform and ultimately a socialist state. It has rejected suggestions by some lawmakers that talks could be arranged between it and the government.

The EPR made headlines in the mid-1990s with a string of lethal ambushes on rural police and army bases.

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