A mother and her boyfriend are wanted by police after leaving her three-year-old son at hospital so badly beaten that he has been left visually impaired.

The child's body was covered in burns and bruises, both old and new injuries, authorities said when he was brought in last Wednesday. 

Jessica Linscott, 23, and her 27-year-old boyfriend Roland Dow were expected to hand themselves in to police on Monday but are now on the run after disappearing in Plaistow, New Hampshire.

Linscott had originally told police that the toddler had injured himself but warrants have been issued for the couple's arrest on 11 counts of child endangerment. 

One relative told police they believed Linscott could be in danger with Dow who is not the child's father.

Plaistow Police Lt. William Baldwin told necn.com that he had never seen 'life-threatening' injuries like those the child had suffered.