Mexico's Congress has toughened laws against oil theft, with prison terms of up to 15 years as illegal taps of pipelines rose again and now cost $2.3 billion a year.

The Senate voted 66-21 in favor of the new law late Tuesday, sending it to President Enrique Pena Nieto's desk for his signature.

The law punishes those who steal hydrocarbons, petrochemicals and other oil products, including from pipelines, and those who buy or sell illegally obtained fuel.

Drug cartels, which have diversified their business, have been poking holes in pipelines to pump out oil products.

Thefts have risen in recent years, with more than 5,000 illegal taps discovered in the first 11 months of this year, compared to 3,286 in the same period in 2014.

The new penalties are four years in prison for stealing less than 300 liters of oil or fuel products; four to eight years for 300 to 1,000 liters; eight to 12 years for 1,000 to 2.000 liters; and 15 years for more than 2,000 liters.

Public officials who fail to report fuel thefts face five years behind bars.

Senator Omar Fayad said the thefts amount to $2.3 billion per year.

The leftist opposition said the sanctions were softer than initially planned.

The new law was approved on the same day that the government held the third auction of its historic opening of the energy sector to private investors. All 25 onshore fields on offer on Tuesday were auctioned off, as investors were undeterred by their locations in violent regions where fuel thefts are rampant.