BISBEE — Even as fuel prices are declining in the United States, many drivers here still cross the border into Naco, Sonora, Mexico, to fill up their tanks.

At a Pemex station in Naco, Sonora, 87-octane gas cost 7.62 pesos per liter on Thursday, or about $2.89 per gallon. In comparison, the Circle K station in Bisbee was charging $3.60 per gallon.

The cost of gas in Mexico has stayed relatively unchanged over the past few months, while prices in the United States have actually decreased.

Back in June, the cost in Naco, Sonora, of 87-octane gas was 7.53 pesos per liter, or about $2.86 per gallon. And, at the time, the price in Bisbee was $3.84 per gallon.

Pemex is Mexico’s state-owned petroleum company. Prices are lower in Mexico due to a $19 billion fuel subsidy announced by President Felipe Calderon in May.

Fuel that is purchased in Mexico and brought into the United States must be declared at a port of entry and is potentially subject to duty collection, said Brian Levin, spokesman for U.S. Customs and Border Protection in Tucson.

“If a person purchases gasoline or diesel while in Mexico on other business, the purchase is considered incidental to their trip and may fall under their personal duty exemption, allowed every 30 days,” he states in a press release. “The fuel must be for the vehicle being driven and in tanks being used to operate that specific vehicle.”

But, Levin adds, if someone goes to Mexico for the expressed purpose of purchasing fuel, then it is subject to proper importation requirements.

And, he says, if a person buys fuel for commercial purposes, such as to sell or to use for a business, “it becomes subject to commercial importation requirements and to all applicable requirements not only for CBP, but also for other agencies with statutory or regulatory requirements for fuel importation and transportation.”

Officials in one Mexican border city have begun fining U.S. drivers who cross the border to fill up additional tanks or drums.

In early August, The Associated Press reported that the city of Ciudad Acuna, which is across the border from Del Rio, Texas, fined four U.S. residents for carrying extra diesel.

Carrying fuel containers across the border violates customs regulations and possibly safety rules. The fines equaled 70 percent of the value of the desiel confiscated. At the time, diesel fuel was selling for about $2.25 per gallon, which was half of the U.S. price.

Last week, an attendant at a Pemex station in Naco, Sonora, said many Americans still frequently cross the border to buy fuel.

But an official at the town hall in Naco, Sonora, said last week that he is not aware of any instances of people buying extra tanks and getting fined for it.