Spain said on Friday all four suspected Ebola cases hospitalised on Thursday, including a Nigerian passenger taken from an evacuated plane by police-escorted ambulance, have tested negative for the deadly disease.

The three other people were a priest who had recently been in Liberia, a man recently in Sierra Leone who tested positive for malaria and a person who traveled in the same ambulance as Teresa Romero, Spain's only known Ebola sufferer.

Spain is on high alert for the disease after Romero, a nurse who cared for two Ebola-infected priests before they died, became the first person to contract the virus outside West Africa. She is gravely ill but stable.

Although all four people tested negative for the disease initially, they must have another test within 72 hours in order to be given the all clear, the government said.

Nearly 4,500 people have died in the current outbreak, practically all of them in West Africa, out of a total of 8,997 confirmed, probable, and suspected cases have been reported in seven countries.

The United States, which is deploying up to 4,000 troops to West Africa to help contain the disease, has asked Madrid for permission to use U.S. military bases in Spain in its operation, a Spanish Defence Ministry source said on Thursday.

Spanish Defence Minister Pedro Morenes meets his U.S. counterpart Chuck Hagel in Washington on Friday to discuss the U.S. request to use the bases at Rota near Cadiz and at Moron de la Frontera near Seville in southern Spain.