The European Union demanded answers about how the disease spread in a specialized disease unit while health staff protested over safety failures.

The nurse worked at Madrid's La Paz-Carlos III, where she cared for two elderly Spanish missionaries who died from the virus after being flown home from West Africa, where the disease has killed nearly 3,500 people.

The Madrid nurse, identified by Spanish media as a woman in her forties called Teresa, became the first person to contract the disease outside Africa.

Officials said they were trying to find out who she came into contact with before being isolated on Monday. They were monitoring 52 people -- mostly health staff.

Doctors at the hospital said her husband was also at "high risk" and was put in isolation. Another "suspect case" -- a Spanish engineer recently returned from Nigeria -- was also being monitored.

A fourth patient, one of the nurse's colleagues, who had suffered from diarrhea, has also been taken in for observation, the hospital said.

The infected nurse had treated Spanish priest Miguel Pajares, 75, who was infected with Ebola in Liberia and died on August 12, as well as Manuel Garcia Viejo, 69, who was repatriated from Sierra Leone and died on September 25.

She is believed to have contracted the virus while caring for Garcia Viejo.

US President Barack Obama warned on Monday that "some large countries are not doing enough" to tackle the epidemic.

Medical staff protested outside the main site of La Paz hospital in their white coats, yelling for Health Minister Ana Mato to resign.

Health worker unions complained staff had not been adequately trained.

Ebola causes severe fever, vomiting, diarrhoea and sometimes internal and external bleeding. It spreads through contact and bodily fluids.

There is no vaccine and no widely available cure, but one treatment, dubbed ZMapp, has shown promising early results and has been fast-tracked for mass production.

Ebola has caused 3,439 deaths out of 7,478 cases across five West African nations -- Liberia, Guinea, Sierra Leone, Nigeria and Senegal -- according to the latest WHO tally.

A specialized Ebola inhumation team bury the body of a recent Ebola victim, on October 6, 2014 in Magbonkoh.

Patients have been treated for the disease in Europe and the United States, but until now all the cases stemmed from people who caught the virus in West Africa.

The International Monetary Fund warned in a report on Tuesday that economic damage from the Ebola outbreak could spread beyond west Africa and become global.

The UK announced on Tuesday that it will send more than 100 army medics to Sierra Leone to help tackle the Ebola crisis.

The World Health Organization on Tuesday convened an informal consultation group on the Ebola crisis, gathering a group of renowned experts in Geneva to discuss the current scientific findings and information emerging from the Ebola-affected nations.

Just hours after Europe's first local case of Ebola infection was confirmed in Spain, the WHO's European director, Zsuzsanna Jakab, said further such events were "unavoidable".