Image copyright Getty Images Image caption The drones pick up on small signs of human disturbance such as footprints and broken twigs

The US government is using drones to patrol half of its border with Mexico, a report by the Associated Press says.

The strategy means that the US is increasingly able to move away from using large numbers of border patrol agents along the entire frontier.

The drones allow border control agents to focus on areas of "greater threat", says the report.

The US border immigration system is under pressure in the face of a worsening border crisis.

According to an investigation by the news agency, there have been about 10,000 drone flights since the new border control strategy began in March 2013.

The unmanned drones are being deployed in an effort to control 900 miles (1,450 km) of remote areas, allowing border patrol agents to focus their resources elsewhere, AP says.

Richard Gil Kerlikowske, the commissioner of the Border Patrol's parent agency, Customs and Border Protection, said his agency only had "finite resources".

"You want to deploy your resources to where you have a greater risk, a greater threat.''

Image copyright Reuters Image caption Many children are trying to cross the border to the US alone

The drones focus on detecting small changes in the landscape such as footprints, broken twigs and tyre tracks.

A border control agent is only sent to the area if the drone has picked up signs of human disturbance, said AP.

Drone patrols are expected to expand to the Canadian border in 2015, the news agency added.

Michael McCaul, a Texas Republican who chairs the House Homeland Security Committee, praised the approach but said that surveillance gaps still remain.

"We can no longer focus only on static defences such as fences and fixed [camera] towers,'' he said.

Civil liberties groups have previously expressed concerns over the use of domestic drones for surveillance, arguing that it could raise privacy concerns.

President Barack Obama is under pressure to fix the immigration crisis growing on the south-west border of the United States.

Over 50,000 unaccompanied children were caught trying to cross the border from Central America since October 2013.