WASHINGTON — U.S. drones deployed along the border are grounded most of the time, cost far more than initially estimated and help to apprehend only a tiny number of people trying to cross the border illegally, according to a federal audit released Tuesday.

In a report that could undermine political support for adding more drones to secure the nation’s borders, the Department of Homeland Security’s inspector general found “little or no evidence” that the existing fleet had met expectations or was effective in conducting surveillance.

U.S. Customs and Border Protection has been flying surveillance drones for nearly a decade, launching them from bases in Texas, Florida, North Dakota and Arizona. It currently has nine Predator B models and has plans to more than double the size of its drone fleet to 24 as part of a $443 million expansion.

The inspector general, however, questioned whether those plans make any sense or would be cost-effective.

In an audit of the fleet’s operations during the 2013 fiscal year, the inspector general calculated that it cost $12,255 per flight hour to operate the drones, a figure five times higher than estimates by Customs and Border Protection.

Although the agency planned to fly four drone patrols a day, the aircraft were in the air for less than a quarter of that time, the audit showed. Hindering operations were a lack of personnel, spare parts and bad weather.

As evidence, the report cited statistics showing that of the 120,939 illegal border crossers apprehended in Arizona during 2013, fewer than 2 percent were caught with the help of drones providing aerial surveillance.

In a written response to the audit, Eugene Schied, an assistant commissioner with Customs and Border Protection, disputed the characterization of the findings. The drone program, he said, “has achieved or exceeded all relevant performance expectations.”

Schied accused the inspector general of cherry-picking statistics and ignoring ones that make the drones appear more effective.

For instance, he said, drones “directly contributed” to the seizure of almost 50,000 pounds of marijuana along the southwest border in 2013, worth an estimated $122 million.