A U.S. Border Patrol agent watches as people who've been taken into custody related to cases of illegal entry into the United States, stand in line at a facility in McAllen, Texas. Picture: U.S. Customs and Border Protection's Rio Grande Valley Sector via AP

Washington - The US Department of Homeland Security said late Saturday it had reunited 522 children removed under its "zero-tolerance" initiative with adults.

A further 16 children will be reunited on Sunday, the department said. A small number of children, who were separated for other reasons, such as being unable to establish familial relationships, will remain separated, it added.

As of June 20, 2,053 separated children were being cared for by Department of Health and Human Services in its facilities.

President Donald Trump on Wednesday caved to pressure and signed an executive order to keep migrant families together when they cross the US-Mexico border, after details about the separations triggered widespread outrage.

Announced in April, the "zero-tolerance" policy meant children were separated from their parents, who were immediately referred for prosecution after they illegally crossed the border.

The children were left in the custody of a sponsor or in shelters.