The Pentagon is using high-altitude balloons to conduct surveillance test in six Midwestern states, the Guardian newspaper reports.

The U.S. military is using up to 25 of the solar-powered balloons to surveil vehicles in the Midwestern states of South Dakota, Minnesota, Iowa, Wisconsin, Missouri and Illinois, according to documents from the Federal Communication Commission.

The surveillance balloons are carrying small satellite-like units with synthetic aperture radars, sensors and communications equipment, the Guardian reports.

The balloons travel at altitudes of up to 65,000 feet and are able to track multiple vehicles at a time, day or night, in all kinds of weather.

The documents reveal that the tests are being conducted to "provide a persistent surveillance system to locate and deter narcotic trafficking and homeland security threats."

The balloons use an advanced mesh networking technologies which allows them to share data and communicate with one another, reports the Guardian. They are also able to relay the data to receivers on the ground. Unlike surveillance planes or drones, the balloons can stay aloft for weeks or months.

According to the documents, the U.S. military has acquired a license to operate the surveillance balloons from mid-July until September.

The tests are being commissioned by the U.S. Southern Command, or Southcom, which is responsible for disaster response, intelligence and security operations in the Carribbean and Latin America.

“We do not think that American cities should be subject to wide-area surveillance in which every vehicle could be tracked wherever they go,” Jay Stanley, a senior policy analyst at the American Civil Liberties Union, told the Guardian.

“Even in tests, they’re still collecting a lot of data on Americans: who’s driving to the union house, the church, the mosque, the Alzheimer’s clinic,” he said. “We should not go down the road of allowing this to be used in the United States and it’s disturbing to hear that these tests are being carried out, by the military no less.”