WASHINGTON — U.S. Customs and Border Protection’s drones have had “minimal” impact on stemming illegal immigration, and the agency has drastically understated the program’s cost while failing to prove its value, an inspector general’s report said on Tuesday.

The Department of Homeland Security’s inspector general recommended the agency scrap plans to spend $443 million on additional unmanned aircraft systems, suggesting it put the money to better use.

“Notwithstanding the significant investment, we see no evidence that the drones contribute to a more secure border, and there is no reason to invest additional taxpayer funds at this time,” Inspector General John Roth said in a statement on the report titled “High Costs, Low Returns for CBP Drone Program.”

In a scathing statement that began “After spending eight years and hundreds of millions of taxpayer dollars,” the Office of the Inspector General said its second audit of the program since 2012 found Customs did not yet have a reliable way to measure the program’s performance.

It said drone surveillance helped with fewer than 2 percent of captures of people crossing the border illegally during fiscal year 2013.

While Custom’s Office of Air and Marine calculated the cost of operating a drone at $2,468 per hour, the inspector general’s office put the actual hourly rate at $12,255. Tuesday’s report said the agency’s estimate did not include key expenses, such as salaries for operators and equipment.

“We disagree with the report, and we feel it inaccurately portrays the program’s effectiveness,” Customs and Border Protection spokesman Carlos Lazo said. He said the agency was not expanding the program, as the report suggests, and aimed to keep it at 10 aircraft.

The inspector general said Customs “has touted drone surveillance of the entire Southwest Border (1,993 miles from Texas to California),” but coverage was actually limited to a 100-mile stretch in Arizona and 70 miles in Texas.

The report also said the Office of Air and Marine failed to meet its flight goal of 16 hours per day, 365 days per year. The drones, often grounded by weather, “were airborne only 22 percent of those goal hours.”

Customs temporarily grounded its drone fleet in January 2014 after a crew had to crash a pilotless craft off the Southern California coast because of a mechanical problem.

Lazo said the agency had nine unmanned planes and would replace the one lost last year.