Reporters and Democratic lawmakers have been allowed inside a detention centre that lies at the heart of a growing storm over a new US policy separating migrant children from their parents.

Authorities did not allow photos or videos to be taken inside the centre, but US Customs and Border Protection later released several images.

The Texas facility is known as Ursula, though immigrants are reportedly calling it La Perrara - dog kennel in Spanish - in reference to the cages used to hold children and adults who have ended up there after crossing the border from Mexico illegally.