US officials have ordered DNA tests on "under 3,000" detained children who remain separated from their migrant parents, in an effort to reunite families at the centre of a border crisis, a senior official has said.

The Department of Health and Human Services is "doing DNA testing to confirm parentage quickly and accurately," its Secretary Alex Azar told reporters on a conference call.

He stressed that the department is seeking to meet a court-imposed deadline of next Tuesday to reunite some 100 detained children aged under five.

Mr Azar, in a bid to deal with accusations that President Donald Trump's administration has failed to account for some minors, said that: "HHS knows the identity and location of every minor in the care of our grantees".

He added that authorities are working to reunite children with their parents "as expeditiously as possible."