A Spanish nurse has become the first person to contract Ebola outside of Africa, casting doubt over measures taken in Spain to control the potential spread of the deadly disease.

The nurse had helped to treat two priests who contracted Ebola in Africa and were repatriated to Spain. Some 30 other health workers and those who came in contact with her are now being monitored for symptoms.

Both priests died shortly after reaching Spain. Each had worked in West Africa, where an epidemic of Ebola has spread through Guinea, Sierra Leone and Liberia since March, killing more than 3,400 people in the largest outbreak of the disease in history. Cases have also reached Senegal and Nigeria.

Spanish officials said they still had to find out how the nurse, who was not named but identified as married with no children, contracted the viral infection, which causes fever and bleeding.

"At the moment we are investigating the way in which the professional was infected," Antonio Alemany, the head of Madrid's primary health care services told a news conference.

The nurse was one of a specialist team who treated elderly priest Manuel Garcia Viejo at the Madrid hospital Carlos III when he was repatriated from Sierra Leone with Ebola on Sept 21. He died four days later.

Garcia Viejo was kept in isolation during his treatment last month and officials said they followed a strict protocol designed to protect health workers and patients at the hospital.

The nurse who has since fallen ill only entered Garcia Viejo's room twice, once after his death, Alemany said.