(NewsNation) — The Department of Homeland Security is quietly removing a strategic tool that agents say is critical in their mission to secure the southern border.

Last year, there were at least 17 surveillance blimps, known as aerostats, along the U.S.-Mexico border. Now, a Department of Homeland Security source tells NewsNation there are only six in operation. ​

To border patrol agents, it is a reduction in support, as the aerostats are used to detect drones and people trying to remain undetected.

Agents are now concerned that the number of people trying to evade officials will only increase.

Per Department of Homeland Security sources, since October, more than 300,000 people have successfully crossed into the U.S., avoiding law enforcement as they did so.

According to U.S. Customs and Border Protection, these blimps are equipped with day and night cameras to provide persistent, low-altitude surveillance, allowing Border Patrol agents to maintain visual awareness of border activity for long periods of time.  

Art Del Cueto, vice president of the Border Patrol Council, says the blimps act as a deterrent as well, because if the cartel can see them, they won’t push people or drugs across the border.

“I think we need to work with everything that we have, use other tools, whether they are to deter, to apprehend, to detect,” he said. “But unfortunately, we’re under an administration now that’s perfectly comfortable removing one tool, (and) adding the different tool instead of saying ‘Hey, we can utilize both tools at the same time and be more effective at defending our borders.’”

From fiscal year 2014 through fiscal year 2020, the aerostat systems were responsible for detecting 68% of all suspected air-smuggling flights approaching the southwest border from Mexico. While there is a reduction in blimps along the southern border, Customs and Border Patrol says it has plans to expand deployments of the aerostats to the coastal sector. In a statement, CBP says it is increasing the number of autonomous surveillance towers along the border as well.

National security experts say the aerostat balloons are extremely expensive to operate, which is one reason they are being reduced. 

News of fewer aerostats comes as border officials testified this week about the pressure they’re facing on the job.

Grande Valley Sector Chief Patrol Agent Gloria Chavez said at a recent Congressional hearing that in the Rio Grande Valley sector of Texas alone, Border Patrol has faced over 10,000 drone incursions and 25,000 drone sightings in just one year.