There is evidence the numbers of dead and sickened by Ebola in West Africa may "vastly underestimate the magnitude of the outbreak," the World Health Organization said Thursday. The U.N. health agency said it was prepared for the crisis to continue for months.

With more than 1,060 deaths and 1,975 sickened, the Ebola outbreak is already the deadliest ever.

"Staff at the outbreak sites see evidence that the numbers of reported cases and deaths vastly underestimate the magnitude of the outbreak," WHO said.

Liberian officials faced a difficult choice Thursday: deciding which handful of Ebola patients will receive an experimental drug that could prove life-saving, ineffective or even harmful.

ZMapp, the untested Ebola drug, arrived in the West African country late Wednesday. A day later, no one had yet received the treatment, which officials said would go to three people.

The outbreak, which was first identified in March in Guinea and since spread to Liberia, Sierra Leone and Nigeria, has overwhelmed the already strained health systems in West Africa and raised questions about whether authorities are doing enough to respond.

There is no licensed treatment for Ebola, a virus transmitted by contact with bodily fluids, so doctors have turned to the limited supply of untested drugs to treat some cases.

The Liberian government had previously said two doctors would receive ZMapp, but it was unclear who else would. Information Minister Lewis Brown said Thursday it would probably be another health care worker.

These are the last known doses of ZMapp left. The San Diego-based company that developed it has said it will take months to build up even a modest supply.

The outbreak has sparked an international debate over the ethics of giving such untested drugs to the sick and of deciding who should get the drugs. So far, only two Americans and one Spaniard have received ZMapp. The Americans are improving — but it is unclear what role the drug has played. The Spaniard died within days.

Now Liberian officials are facing those questions. In this outbreak, over 50 per cent of those sickened with Ebola have died, according to the UN health agency.

"The criteria of selection is difficult, but it is going to be done," said Dr. Moses Massaquoi, who helped Liberia obtain the drug from Mapp Biopharmaceutical. "We are going to look at how critical people are. We are definitely going to be focusing on medical staff."

'Who gets the chance to be experimented on?'

He added people past the "critical phase" who looked likely to survive would not be chosen.

Massaquoi said there was only enough of the drug to treat three people. Treatment will be staggered, so doctors can observe the effects in one patient before moving on to the next. Late Thursday, he said the treatment had not yet started.

Arthur Caplan, director of medical ethics at NYU Langone Medical Center, said the choice of who to treat would have to balance helping the largest number of people with learning the most from the treatments.

He said the question is not "whose life do we save?" but "who gets the chance to be experimented on?"

For that reason, recipients need to be good experimental subjects — people who have recently contracted the disease and are more likely to respond to treatment or perhaps younger patients, he said. In order to study the long-term effects, doctors will likely prefer people who can be observed for months, which might eliminate those living in remote places, he added.

Meanwhile, the U.S. State Department ordered families of embassy personnel to leave Sierra Leone on Thursday because of concerns that the crisis would make it difficult to get treatment for even routine health problems.

President Barack Obama spoke by phone Thursday about the Ebola outbreak with Liberian President Ellen Johnson Sirleaf and in another call with President Ernest Bai Koroma of Sierra Leone. The White House said Obama expressed his condolences for the hundreds who have died in both countries and underscored the U.S.'s commitment to work with West African nations and U.N. agencies to contain the outbreak.

Also, Nigeria announced Thursday that another person had died from Ebola, bringing the country's death toll to four. The Health Ministry said the person was a nurse who helped treat the country's first Ebola case, Liberian-American Patrick Sawyer who flew in last month and died.

The ministry corrected its total number Ebola cases to 10 instead of 11 as it had reported earlier in the day.