More than 60 per cent of Mexico's steel production was halted and two major auto plants, including Volkswagen's only manufacturing facility in North America, shut down Tuesday after explosions claimed by a leftist group cut natural gas supplies.

Petroleos Mexicanos said the attacks on its oil and natural gas pipelines would cause hundreds of millions of dollars in production losses for the state-owned oil company and affect 10 states. Private-sector groups told Mexican news media that the attacks and subsequent precautionary shutdowns would cost businesses close to US$90 million.

Mexico's steel industry chamber Canacero said Tuesday that more than 60 per cent of the country's steel production has been halted, and that it could take up to seven days to resume.

In a statement, Canacero said losses would "significantly" exceed the $36 million the industry suffered from similar attacks on pipelines in July.

The six explosions affected a dozen natural gas pipelines and one oil pipeline in the Gulf coast state of Veracruz, sending flames and black smoke shooting into the air but causing no direct injuries. The blasts occurred at valve stations where different pipelines intersect.

Industry and national-security experts say the small leftist group claiming responsibility has proved it is a force to be reckoned with.

"The sophistication required to plan, co-ordinate and execute these explosions shows that the perpetrators have the technical capability of turning these episodes into either terrorist attacks or industrial sabotage," George Baker, a Houston-based energy analyst who follows Petroleos Mexicanos, or Pemex, wrote in a report.

The Revolutionary People's Army, or EPR, a secretive Marxist group that killed dozens of police and soldiers during attacks in the late 1990s, claimed responsibility for the explosions in a statement e-mailed to various news organizations on Tuesday, the Reforma and El Universal newspapers reported on their websites.

The organization said it had placed 12 explosive devices, including one that did not detonate, on an equal number of pipelines and would continue to do so to demand the release of two group members missing since May. It also said it was protesting President Felipe Calderon's alleged repression of political opponents, human rights defenders and "social fighters."

The group made similar statements in July, when it claimed responsibility for explosions along a pipeline between Mexico City and Guadalajara.

The EPR, which says it is waging a "prolonged people's war" against "the anti-people government," had been weakened by internal divisions and was largely inactive in recent years.

"But something has changed because now they have the capacity to attack pipelines," said Mexican national-security analyst Jorge Chabat. "They're acting outside their regular sphere of influence and that is a problem."

Chabat said the group's ability to deal a strong blow to the economy is no small matter. "If they can attack Pemex pipelines, well then yes, you have to take them seriously."

Pemex chief Jesus Reyes said Monday it will take four or five days from the time the company can start repair work for natural gas to be restored.

The gas shutdown forced Volkswagen AG to suspend production at its sprawling car factory outside the central city of Puebla, possibly until Monday. Chrysler LLC's plant in Toluca, outside Mexico City, was also shut Tuesday and could be idled for up to five days, union officials said. Natural gas is used for heating purposes in industrial processes, such as in steel blast furnaces.

Mexican news media reported that more than 1,000 businesses had been affected in 10 states.