Approximately 500 children who were separated from their families under the Trump administration's “zero tolerance” immigration policy have been reunited, officials have claimed.

An administration official told multiple outlets on Friday that all unaccompanied children in the custody of Customs and Border Protection (CBP) who were separated from adults being prosecuted under the new tolerance policy had been reunited.

But the number of children returned to their families was far from the approximately 2,300 children who had been taken from their parents since the zero tolerance policy began in April. Migrant children only stay in the custody of CBP for a matter of days after they are apprehended, and are then placed in the care of the Department of Health and Human Services (HHS).

HHS said on Friday that it was working on a plan to reunite the children in its care with their families as well.

"Secretary (Alex) Azar is bringing to bear all the relevant resources of the department in order to assist in the reunification or placement of unaccompanied alien children and teenagers with a parent or appropriate sponsor," HHS spokeswoman Evelyn Stauffer told CNN.

Screaming children heard crying for parents at US detention centre after being separated at border under Trump policy, in distressing audio recording

A small number of children in CBP custody were not reunited with their parents immediately, either because a familial relationship could not be confirmed, authorities believed the adult posed a threat to the child, or the adult had been convicted of a crime, the administration official said.

Thousands of children were separated from their parents in recent weeks, after the Trump administration began requiring all adults caught crossing the border illegally to be referred for prosecution.