(Adds Pemex statement, paragraphs 7-8)

By Frank Jack Daniel and Alistair Bell

MEXICO CITY, Sept 11 (Reuters) - A leftist rebel group claimed responsibility on Tuesday for bomb attacks on Mexican oil and gas pipelines earlier this week, and threatened more assaults against the state-owned oil company.

Blasts at six pipelines on Monday disrupted oil and gas supplies, forced thousands from their homes and caused hundreds of millions of dollars in damage to oil monopoly Pemex and other businesses. No one was injured.

It was the second such attack in two months and the Popular Revolutionary Army, or EPR, rebel group said it planned more. The group accuses the government of secretly abducting two of its guerrilla organizers.

"Our political-military activities will not be stopped until our two abducted or disappeared companions are shown free and alive," the rebels said in a statement sent to a Mexican newspaper. They promised to avoid casualties.

The EPR shocked Mexico when it emerged over a decade ago with a series of lethal attacks on army and police outposts but had been quiet for years until it emerged again in July.

Huge billowing flames leaped into the sky after Monday's blasts and more than 20,000 people were rushed from their homes to emergency shelters as gas and oil spewed from the ruptured pipes.

Some 750 Mexican engineers started repairs on four natural gas pipelines blown up in the attacks and were close to extinguishing a fire at one of the fuel ducts on Tuesday evening, Pemex said.

Pemex said in a statement it hoped domestic natural gas supplies would be back to normal by early on Monday, Sept 17.

The bombings halted 60 percent of Mexico's steel industry and affected more than 2,5000 companies, causing daily losses of $100 million while gas supples are cut, business groups said.

RURAL GUERRILLAS

The EPR's new strategy of bomb attacks on energy pipelines began in July with a wave of bombings that cut natural gas supplies to thousands of businesses. Monday's bombings were on an even larger scale.

The EPR burst into public view in 1996 when about 70 masked and armed men strode onto a stage at a rally in the poor southern state of Guerrero. The rebels fired 17 shots in the air, one for each peasant killed in an infamous police massacre there a year earlier.

At the time, the group claimed 500 members across Mexico. Within months it launched a string of lethal ambushes on rural police and army bases, culminating in two days of coordinated attacks in several states. Dozens of police and soldiers were killed.

The group set off a bomb at a Sears department store in Oaxaca in August and attacked a jail under construction in the southern state of Chiapas in July.

Mexico has deployed soldiers and federal police to protect pipelines but President Felipe Calderon has acknowledged it is almost impossible to completely secure the vast network.

Mexico is a major exporter of crude to the United States but its six refineries were operating as normal and exports of oil, gas and fuel products were not affected, Pemex said.

German automaker Volkswagen suspended production at its plant in Puebla state in central Mexico, due to a lack of natural gas following the bombings.

The plant is Volkswagen's sole factory in North America and produces the New Beetle.

(Additional reporting by Catherine Bremer and Robin Emmott)

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