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 top US official said today.

The Department of Health and Human Services is "doing DNA testing to confirm parentage quickly and accurately," HHS Secretary Alex Azar told reporters on a conference call, stressing the department was seeking to meet a court-imposed deadline of next Tuesday to reunite some 100 detained children under age five.

Azar, in a bid to tamp down 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," and that authorities were working to reunite children with their parents "as expeditiously as possible."