(Adds Pemex chief executive quotes, detail, background)

By Catherine Bremer

MEXICO CITY, Sept 11 (Reuters) - Mexican state oil monopoly Pemex will send engineers on Tuesday to start repair work on four natural gas pipelines blown up in sabotage attacks the previous day, but fires still raged at two other fuel ducts.

Pemex Chief Executive Jesus Reyes Heroles told Mexican television he hoped domestic natural gas supplies would be back to normal by the weekend.

Pemex is set to lose millions of dollars after six pipeline attacks in southeastern Mexico cut off a quarter of its natural gas flow, halting supplies to businesses in the area.

"I think immediate repair work will start today in four (locations)," said David Lartundo, a Pemex spokesman in Veracruz state, where most of the explosions occurred.

"Already yesterday we had various points where the flames were out ... and this will allow the repair teams to go in today and start to take the necessary measures to reestablish the service as soon as possible," he told Reuters.

Monday's explosions, attributed to a leftist rebel group which carried out a spate of similar attacks in July, did not affect Mexican oil exports but forced thousands of people out of their homes and left nearby factories struggling to operate.

The ruptures affected a dozen pipelines in all, mostly natural gas ducts but also pipes containing liquid petroleum gas, petroleum products and crude oil headed for a refinery. Pemex said the refinery had enough inventory to run as normal.

German automaker Volkswagen <VOWG.DE> suspended production at its Puebla plant, its sole North American factory, on Tuesday. Mexican glass maker Vitro <VITROA.MX> and industrial conglomerate Grupo Industrial Saltillo (Gissa) <GISSA.MX> shut down several manufacturing plants on Monday.

Reyes Heroles said state-owned Pemex was losing $3 million per day in lost natural gas sales, but it was too early to quantify the cost of repairing the ruptured pipelines. On Monday he said the explosions would cost Pemex "hundreds of millions of dollars" in total.

INEVITABLE VULNERABILITIES

Known by its Spanish initials EPR, the Popular Revolutionary Army left a written message beside a bomb that failed to explode on Monday, Mexican daily El Universal said.

A local government official in Veracruz said the authenticity of the note was still in doubt, however.

Mexico has deployed army and police guards along its vast network of pipelines, which stretches for more than 14,000 km (8,700 miles), but officials all the way up to President Felipe Calderon admit it is impossible to completely secure it.

"The truth is that a company with the characteristics and size of Pemex inevitably has vulnerabilities that are difficult to eliminate," Reyes Heroles said on Tuesday, estimating the mammoth oil company has some 19,000 onshore installations.

He said the most important sites had 24-hour security, but minor installations were only checked periodically.

Lartundo said Pemex's security system involved high-tech monitoring on top of surveillance patrols, but said no system is infallible. Mexico's six refineries, being on contained sites, are easier to guard, he said.

He said Pemex may look at rerouting some natural gas via other pipeline networks to lessen the impact to industrial clients while engineers work to repair the ruptured ducts.

"We have two points today which are still on fire," he said. "Both are confined areas. The fire has been left because it's burning the remnant of the fuel that was in the pipes. Once it's burned off the maintenance teams can go in."

Mexico is the world's No. 5 oil producer and a major U.S. supplier. It is a net importer of natural gas and gasoline, although it sells a small amount of domestic natural gas and gasoline abroad along with fuels like diesel and jet fuel.

Pemex itself is a major consumer of natural gas, used to run production wells, but it normally pumps the gas up from the same field it is drilling.

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