Acting US Defense Secretary Pat Shanahan says the Pentagon will be sending “several thousand” more American troops to the US-Mexico border, where about 2,350 active-duty troops were deployed last year.

Shanahan said Tuesday the troops will mainly be used to install additional wire barriers and provide a large new system of mobile surveillance and monitoring of the border area.

The Department of Homeland Security has asked for additional help putting up fencing and with “expanded surveillance,” the Pentagon chief said.

The US Defense Department estimates that the total number of forces deployed to the border over the coming months will exceed 4,000.

The controversial border mission was supposed to wrap up January 31 but has been extended through September.

Shanahan’s comments came as members of the House of Representatives Armed Services Committee grilled top military officials about the border mission, demanding details on its impact on military readiness and whether needed training and other jobs were not being done as a result.