A consignment of experimental Ebola drugs has arrived by plane in Liberia to treat two doctors suffering from the virus, which has killed more than 1,000 people across four West African countries.

The drug, ZMapp, arrived in two boxes on a commercial flight from the United States carried by Liberia's foreign affairs minister, Augustine Ngafuan, and was unloaded at the VIP terminal, a Reuters witness said.

It will be taken to a hospital in the capital and administered to Liberian doctors Zukunis Ireland and Abraham Borbor, who officials said contracted the disease while attending to patients, including a late colleague.

The world's worst outbreak of Ebola has claimed the lives of 1,069 people and there are 1,975 probable and suspected cases - the vast majority in Guinea, Liberia and Sierra Leone - according to new figures from the World Health Organisation (WHO).

Three people have died in Nigeria.

The UN health agency says only about 10 to 12 doses of the drug have been made and this raises difficult ethical questions about who should get priority access.

What is ZMapp? The experimental Ebola treatment was developed by US company Mapp Biopharmaceutical based on years of US and Canadian research. Read more.

The doctors will be the first Africans to receive it, although it has been given to a Spanish priest, who later died, and two US aid workers who are reported to have shown signs of recovery.

Authorities are also concerned that ZMapp's unproven status could leave them open to the charge that humans are being used as guinea pigs.

"This is not the panacea to the problem. It is at the risk of the patient," Liberia's assistant health minister, Tolbert Nyenswah, told journalists at Monrovia's main airport.

Information minister Lewis Brown told Reuters the drug merely offered a "glimmer of hope" and its use was little more than a gamble.

Even so, the clamour for it is strong given the contagious haemorrhagic disease is killing more than half of its victims and there is no known cure or vaccine.

"I welcome it. It is very good. Our nurses are dying. If you bring them the medication it will make them stronger to fight Ebola," Monrovian stationery seller James Liburd said.

Sierra Leonean doctor dies

In evidence of the ethical dilemma, Melvin Korkor, the first Liberian doctor to survive Ebola, said he would not have used ZMapp when he was fighting for his life because US authorities said they were not responsible for any adverse effects.

"Any drug that has not been approved by [the] FDA should not be administered," he told Reuters.

One of the epidemic's most tragic consequences is the toll on healthcare workers, who rushed in only to become infected themselves due to inadequate protection measures or diagnoses of patients that came too late or were inaccurate.

The WHO said this week 170 healthcare workers had been infected and at least 81 had died.

Sierra Leonean doctor Modupeh Cole became the latest medical practitioner to die of Ebola, a health ministry spokesman said on Tuesday.

He contracted the disease after treating a patient who later proved to have the virus and died. The country's leading Ebola doctor, Shek Umar Khan, also died last month.

Eight Chinese health workers are in quarantine in Sierra Leone because they may have contracted Ebola, according to the spokesman for the Chinese embassy in Freetown, Xu Zhou.

The seven doctors and one nurse treated patients at two Chinese-run hospitals in Freetown who later died from Ebola.

One of the doctors has emerged from quarantine after a 21-day observation period.

Guinea declares 'state of emergency'

West African and other governments, including some which have seen no cases of the virus, have taken measures intended to prevent the spread of the disease.

Guinea-Bissau has decided to close its frontier with eastern neighbour Guinea, prime minister Domingos Simoes Pereira told a news conference.

Germany on Wednesday urged its nationals to leave Liberia, Guinea and Sierra Leone, although the request did not apply to medical workers or German diplomatic staff, a foreign ministry spokesman said.

In Guinea, the president has declared what he calls "a national health emergency" over the ebola crisis.

The death toll there is about 380.

The outbreak has brought fresh attention to efforts to find a cure.

Scientists in the US say they have found how it blocks and disables the body's ability to battle infections in a discovery that should help the search for potential cures and vaccines.

The scientists found that Ebola carries a protein called VP24 that interferes with a molecule called interferon, which is vital to the immune response.

Reuters