By Catherine Bremer

MEXICO CITY, July 11 (Reuters) - Extra soldiers and federal police were deployed to guard Mexican oil wells and pipelines on Wednesday after a rebel group said it caused explosions at fuel pipelines as part of an anti-government campaign.

State oil monopoly Pemex, a major U.S. supplier, said it beefed up its police and army presence at installations after the left-wing Popular Revolutionary Army, or EPR, said on Tuesday it was behind attacks on four pipelines in the past week. It vowed to carry out further attacks.

Pemex added two extra helicopters to its surveillance operations and was working with the defense ministry and federal police on the best way to guard the country's pipelines, which stretch over 14,000 km (8,700 miles).

"What we are doing is reinforcing this presence, particularly in our network of pipelines," a spokesman said.

Mexico is the world's No. 9 exporter of crude oil and valued by the United States as a politically stable supplier.

Police were also deployed to guard strategic locations in Mexico City, after the federal government reacted to Tuesday's threat by saying it will ramp up national security.

"They are guarding strategic points in various places," a spokesman for the city's security ministry said.

Pipeline blasts on Tuesday and last Thursday hit domestic supplies of natural gas, liquefied petroleum gas, crude oil and gasoline in central Mexico.

Thousands of local residents were evacuated but there were no injuries, and no impact on Mexico's crude oil exports.

Federal police are still investigating the explosions, which all occurred at around 1 a.m. local time, but the government said it was taking the threat seriously.

The Marxist-inspired EPR emerged in 1996 as a guerrilla group active in the poor southern states of Guerrero, Michoacan, Oaxaca and Chiapas.

The group has been quiet, mainly conducting its campaign via statements on the Internet, since 2001 when a splinter group known as FARP set off small homemade bombs outside three banks in Mexico City. But its initials were spray-painted on walls near Tuesday's pipeline explosion.

The EPR said in a statement it opposed the government of conservative President Felipe Calderon, whose razor-thin 2006 election win was contested for months by leftists claiming fraud.

The group said it would continue its campaign until the government releases three activists arrested in May in the politically turbulent state of Oaxaca.

However, Mexico's attorney general's office said on Wednesday that as far as it was aware the three men were not in prison.

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