Yarelis Bonilla, five, of Elizabeth in New Jersey, has lymphocytic leukaemia and her best hope is a bone-marrow transplant from a perfect match donor.

Her seven-year-old sister, Gisselle, would fit the bill but she lives in El Salvador and has twice been denied a visa by the US embassy.

Marian Habib, the lawyer representing the family of Yarelis, who was born in the US, said: “It is truly an emergency. This is a race against time.” She said the US embassy in El Salvador has to be convinced that the girl’s life is at risk and that her sister will return to her home country after the transplant.