July 10 (Bloomberg) -- A Mexican guerrilla group claimed responsibility for bomb attacks on natural gas pipelines owned by Mexico's state oil company, carried out today and last week.

The group, the Popular Revolutionary Army, said the sabotage will continue until President Felipe Calderon and the governor of the state of Oaxaca, Ulises Ruiz, return alive three missing members. Calderon ordered heightened security at Mexico's ``strategic installations,'' presidential spokesman Maximiliano Cortazar said.

The group's threat, posted to a Web site used by revolutionary organizations, adds to a sense of insecurity created by mounting drug violence that has killed more than 1,300 people this year. Calderon, whose popularity surged following his military crackdown on traffickers, now confronts an armed political group destroying state assets and refusing to recognize his presidency

``Even though this is a small group, the risk is the perception that Mexico is sliding into anarchy,'' said Chappell Lawson, a political science professor at the Massachusetts Institute of Technology. ``Calderon's got to find these guys and get them before they start blowing up other stuff.''

Today's blast in the central state of Queretaro took place at a Petroleos Mexicanos valve-control station in Corregidora at 1:10 a.m. local time, according to the state government. Pemex, as the company is known, said in a statement that its technicians cut the flow of gas through pipelines and the fire was under control. There were no reported injuries.

Exports

Exports of natural gas weren't affected, said Marta Avelar, a Pemex spokeswoman. The other damaged pipelines supply domestic users, she said. ``The problem is local,'' Avelar said.

Former President Vicente Fox sent more than 4,500 federal police to the capital of the state of Oaxaca last year to end a six-month uprising. An umbrella protest group known as the Popular Assembly of the People of Oaxaca, which occupied the colonial center of the city, listed the resignation of Ruiz as its principal demand.

A Pemex gas pipeline in Guanajuato state was hit by two explosions on July 5 that prompted the evacuation of 4,100 people and damaged nearby gasoline and gas-liquids lines. No casualties were reported and shipments were unaffected by the blasts, Avelar said today.

The attorney general's office, Mexico's top law enforcement ministry, said in a statement that its initial investigation of the Guanajuato explosions showed the blasts were intentional because they originated outside the pipeline, were set off simultaneously and had the same technical pattern. The investigation is continuing.

Ministry Probe

The ministry began an investigation into today's blast without any results to report yet. Authorities are also investigating the authenticity of the statements by the armed group that claimed responsibility, the ministry said.

``The results of this investigation and the coordination of government institutions will help bring to justice the persons who attempted to disrupt the stability, peace and the free development of Mexican society,'' according to the statement.

More Violent

The EPR, as the group is know by its Spanish acronym, first appeared in the southern states of Oaxaca and Guerrero two years after the 1994 uprising of the Zapatista Army for National Liberation in the state of Chiapas.

Its members attacked soldiers and set off bombs, distinguishing themselves as more violent than the Zapatistas who avoided conflict with Mexican authorities after their initial uprising in January 1994.

In 1996, as many as 80 members of the Marxist EPR attacked government offices in the Oaxaca beach resort of Huatulco, killing about 10 people. President Ernesto Zedillo increased military patrols and boosted anti-poverty spending.

The EPR used eight bombs to carry out today's and last week's attacks, the group said.

Pemex, based in Mexico City, is the third-largest oil supplier to the U.S.

The EPR statement said the bombings signal the beginning of its campaign against the interests of ``the oligarchy and of this illegitimate government.''

`Illegitimate'

The word ``illegitimate'' echoes presidential contender Andres Manuel Lopez Obrador, who lost the 2006 election to Calderon by less than 0.6 percentage point, and uses the same term for the current administration. After leading two months of post-election street protests culminating in a self- inauguration, Lopez Obrador continues his claim to be the rightful head of state.

His charges of fraud against Calderon and the National Action Party were thrown out by a federal court.

Analysts such as Jorge Chabat, a political science professor at the Center for Economic Research and Teaching in Mexico City, expressed skepticism that the EPR had carried out the bombings.

``This is a kind of attack that this group hasn't carried out before,'' he said. ``If it's true, then the government will have to increase its watch over these sorts of revolutionary organizations.''

In February a Saudi Arabian terrorist group linked to al- Qaeda called for strikes on oil and gas installations in Canada, Mexico and Venezuela. In May, Mexico formed a government committee to gather and share terrorism-related information with other countries.

To contact the reporters on this story: Thomas Black in Monterrey at tblack@bloomberg.net; Patrick Harrington in Mexico City at pharrington8@bloomberg.net

Last Updated: July 10, 2007 23:28 EDT