A British nurse with Ebola was fighting for her life Sunday as two health workers, who also spent time in Sierra Leone, were placed under observation in the US and Germany.
On Saturday, the London hospital where nurse Pauline Cafferkey is being treated, said her health had taken a turn for the worse.
"The condition of Pauline Cafferkey has gradually deteriorated over the past two days and is now critical," the Royal Free Hospital said.
On Wednesday, doctors had said the 39-year-old Scot, who had been working with the charity Save the Children in Sierra Leone, was sitting up in bed, reading and talking to staff from inside her isolation tent in the hospital.
British Prime Minister David Cameron told BBC television  Ebola was "uppermost" in his mind given Cafferkey's condition and said he was "thinking of her and her family".
Cafferkey's doctors said she had agreed to be treated with blood plasma from an Ebola survivor containing virus-fighting antibodies as well as an experimental anti-viral drug.
Cafferkey was volunteering at a British-built treatment centre in Kerry Town, not far from Sierra Leone's capital Freetown, when she contracted the deadly virus.
She was diagnosed in Glasgow on December 29, a day after flying home, and transferred to the Royal Free, which has the only isolation ward in Britain equipped for Ebola patients.
Expert microbiologist Professor Hugh Pennington said she would need luck to survive the disease, which experts still do not fully understand.
"The plasma is probably her best chance of treatment," he said.
In the United States, a US healthcare worker who also spent time in Sierra Leone, was to be placed under observation at a Nebraska hospital after high-risk exposure to the Ebola virus, health officials said.
In Germany, a South Korean medical worker recently working with patients in Sierra Leone was hospitalised and placed in an isolation unit, according to Berlin's Charite hospital.
The patient was displaying no symptoms, but sustained a "slight" cut to the finger while taking the blood of a patient on December 29.
- 'No plan B'
The Ebola outbreak has killed more than 7,900 people out of more than 20,300 cases, according to the latest tally Friday by the World Health Organization.
Almost all the deaths and cases have been recorded in the three west African countries worst hit by the outbreak: Sierra Leone, Liberia and Guinea.
As part of attempts to halt the epidemic, Sierra Leone announced on Sunday the lockdown in the northern Tonkolili district had been extended for two weeks.
It also imposed "additional screening measures" at Freetown International Airport after two workers apparently caught the disease.
Ismail Ould Cheikh Ahmed, the new head of the UN Mission for Ebola Emergency Response, said the world had no choice but to beat back the infection, as he arrived in the Ghanaian capital Accra to assume duty.
"This is a global crisis. We definitely have a difficult time ahead of us, but we can achieve it," he said.
"We have no plan B; we have to get rid of this virus. This is within our reach, but we should not be complacent.
"We need to keep going until we don't have even one case, because even one case is too many," he said.
Ahmed will be visiting Liberia and Sierra Leone this week, and Guinea shortly after.
Ebola can fell its victims within days, causing severe fever and muscle pain, weakness, vomiting and diarrhoea. In severe cases it shuts down organs and causes unstoppable bleeding.
The virus is spread through direct contact with the bodily fluids of an infected person, meaning people caring for the sick are particularly exposed.