MEXICO CITY (Reuters) - Mexican President Andres Manuel Lopez Obrador said on Monday that his crackdown against fuel theft was yielding positive results, even as the intervention sparked severe fuel shortages in parts of the country and long lines of angry motorists.

In a bid to eliminate years of mounting theft, state oil firm Pemex has changed its distribution, triggering shortfalls in at least six states, including Guanajuato, a major car-making hub in central Mexico.

Guanajuato’s state government said that less than one third of the state’s gas stations were open on Monday.

Lopez Obrador told a news conference the government had not established a date for when operations would return to normal, but stressed that supply was not in danger.

“We are changing the whole distribution system, that’s the reason for the shortage. We have enough gasoline,” he said.

Mexican television showed long lines of drivers waiting to fill up in central states as well as Jalisco in the west and Tamaulipas in the north.

Years of fuel theft by criminal groups and others by tapping pipelines and stealing tanker trucks has led to losses totaling billions of dollars for public coffers.

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Lopez Obrador’s government has ordered the armed forces to intervene in Pemex’s facilities, including one refinery.

“The supply will normalize, and at the same time we are going to guarantee that fuel is not stolen,” said Lopez Obrador, who took office in December. “We have seen a reduction in theft like never before ... but we still have work to do.”

Guanajuato’s governor Diego Sinhue told local radio that of the state’s 415 gas stations, only 115 were open. In Leon, Guanajuato’s biggest city with a population of more than 1.5 million, only 7 of 196 stations were open on Sunday, he said.

“Fuel is becoming a serious problem,” said Sinhue, a member of the opposition center-right National Action Party (PAN). “People are really angry about this shortage.”

Sinhue said the army had informed him it had taken control of the state’s Salamanca refinery on Monday morning. There, members of the armed forces were monitoring tankers going in and out of the facility, as well as the pressure of pipelines.

Energy Minister Rocio Nahle offered an apology on Mexican radio for the shortages. Asked when the problem would be fixed, she said it was in the process of being “normalized”.