Media playback is unsupported on your device Media caption Veronica is terrified she'll be separated from her daughter when she seeks asylum at the US border

US officials have ordered DNA tests on some 3,000 migrant children in an effort to reunite families who have been separated at the US-Mexico border.

US Health Secretary Alex Azar said the tests were needed to meet a court deadline to reunite families, as the agency's usual methods were too slow.

Mr Azar, whose agency oversees migrant detention centres, said about 100 of the children were aged under five.

Activists fear the DNA data can be used by the government for other purposes.

Critics also say that the children are too young to consent to a DNA test.

The agency is bound by a court order to reunite children aged four and under by 10 July and children aged five to 17 by 26 July.

Mr Azar said the agency's traditional method of using birth records to match parents and children would take too long, adding that in order to comply with the deadline, normal vetting processes may need to be truncated.

Media playback is unsupported on your device Media caption Drone footage shows a "tent city" immigration centre

Mr Azar added that of the 11,800 minors in the care of the health department, "under 3,000" had been reported as separated from their families.

He noted that some children may have become separated either before or after crossing into the US.

Immigration activists say the that DNA data can be easily abused once it is collected by the US government, and may be used to track a migrant's whereabouts.

Jennifer Falcon with the immigration advocacy group Raices, tweeted that the decision was "further proof the administration has no idea how to reunite families", that were separated after illegally crossing the US border under President Donald Trump's "zero-tolerance" immigration policy.

Media playback is unsupported on your device Media caption Cisary Reynaud has not spoken to his daughter since they were separated

Also on Thursday, CBS News reported that a federal policy loophole allows two child migrant detention centres - the "Tent City" in Tornillo, Texas and a shelter in Homestead, Florida - to evade child welfare inspections required of other shelters.

The Tornillo and Homestead detention centres are on federal land, and are therefore not subject to oversight by the local state government, according CBS News.

Over the previous weekend, and during the Fourth of July holiday on Wednesday, protesters took to the streets to oppose Mr Trump's decision to separate families.

A US immigrant from DR Congo was arrested for attempting to scale the Statue of Liberty in New York, and has been charged with several offences including trespassing.