Three more people have been admitted to hospital in Madrid to be monitored for signs of Ebola, bringing the total number under observation to 15.

Saturday's development came as the Spanish nurse infected with Ebola was reported to be conscious and sitting unaided. Teresa Romero, 44, is the only person known to have caught the virus outside Africa.

None of the 15 others still under observation has been diagnosed with Ebola so far, though the Spanish government is under fire for its handling both of Romero's case and the threat of a wider outbreak of the disease.

Images by the Reuters news agency showed Romero alert and sitting upright in her hospital room with an oxygen mask strapped to her face and responding to the hospital staff attending to her.

She had taken a turn for the worse two days ago, health authorities said, and is still considered critical.

"Teresa Romero's condition has undergone no significant changes and is still serious, but stable," a government Ebola committee said in a statement on Saturday afternoon, addin that she has begun talking.

The latest outbreak of the disease has already killed more than 4,000 people, mostly in West Africa, and Romero's case has raised fears about contagion in Europe and elsewhere.

The UK has said it will start screening passengers for Ebola who enter the country through London's two main airports and by railway from continental Europe.

The US on Saturday began screening travellers from West Africa at New York's John F. Kennedy International airport.

Spain's government tightened Ebola detection protocols on Friday and entrusted Soraya Saenz de Santamaria, deputy prime minister, with the responsibility to respond to the health crisis.

Men treated by Romero died

Romero was diagnosed with Ebola after caring for two priests who had contracted the disease in Africa and were then repatriated to Spain. Both men died, one in August and one in September.

Another nurse who treated one of the priests was released from hospital late on Saturday, after testing negative for Ebola.

A hairdresser, another nurse and a cleaner, all of whom came into contact with Romero, were admitted to the isolation unit at the Carlos III hospital on Friday evening. The 13 who were already under observation included Romero's husband.

An experimental treatment, ZMab, is available in Spain for use in her case, a health source said. However, it was not clear whether she was now being given the drug. She was given antibodies from previously infected patients earlier this week.

The ZMab combination drug, made by Canada-based company Defyrus Inc, is one of the agents used to make ZMapp, another treatment, which was developed by MappBiopharmaceutical Inc.

ZMapp has been used on some Ebola sufferers, a number of whom survived, but available supplies are exhausted.

Hospital authorities and the government declined to comment.

A hairdresser, another nurse and a cleaner, all of whom came into contact with Romero, were admitted to the isolation unit at the Carlos III hospital on Friday evening. The 13 who were already under observation included Romero's husband.