Mexico has been buying American military equipment at breakneck speed to aid its ongoing fight with drug cartels. Over the past two years, the United States has sold more than $1 billion in military supplies to the country, reported the Washington Post.

A once-cold relationship between the U.S. and Mexico has taken on a different tone when it comes to military sales. The increased sales represent a change in that security relationship, sources familiar with the partnership told the Post. Mexico's recent purchases represent a “hundredfold increase from prior years,” said Admiral E. Gortney, commander at U.S. Northern Command -- the military headquarters that deals with Mexico -- in testimony before Congress.

Mexico's haul reportedly included 30 million bullets, numerous Black Hawk helicopters and thousands of Humvees. The buying-spree indicates that Mexico is pushing to modernize its military in its fight against drug cartels.

In the past, Mexico has not been a big U.S. military buyer, instead relying on European and private sellers. Mexican President Enrique Peña Nieto also emphasized that he felt the U.S. was too involved in the drug war when he took office two years ago, according to the Post.

But a late 2013 order of ammunition worth $6 million that was filled by the U.S. in less than 100 days was reportedly a major breakthrough in the relationship. From there, the purchases grew larger -- reaching 2,200 Humvees and 24 helicopters. In total, Mexican spending on defense has tripled since 2006, in a move perhaps meant to boost the country's ability to fight drug cartels that often outgun the military.

Mexico recently made a public promise for justice after the New Generation Jalisco Cartel shot down an army helicopter with a rocket propelled grenade in early May, killing six soldiers. At least 15 others were killed later in a show of force from the cartel, which included shootouts against police and soldiers.

“The full force of the Mexican state will be felt in the state of Jalisco,” said National Security Commissioner Monte Alejandro Rubido to the Televisa network, according to the Guardian. “Satisfactory results will start to be seen very soon.”

Critics of the U.S. sales have accused Mexican security forces of abusing human rights. The Washington Post listed incidents involving college students who went missing last year and allegedly were captured and killed by police, as well as 22 civilians who were killed by the Mexican military just a few months earlier.

A post on the North American Congress on Latin America website read, "the massive militarization represented by billions of dollars of U.S. arms sales to Mexico -- as well as illegal gun trafficking -- is bad news for the many Mexicans devastated by the abuses of police and soldiers."