Three Liberian health workers receiving an experimental drug for Ebola are showing signs of recovery, officials said Tuesday, though medical experts caution it is not certain if the drug is effective.

The World Health Organization said that the death toll for West Africa's Ebola outbreak has climbed past 1,200 but that there are tentative signs that progress is being made in containing the disease.

The three Liberians are being treated with the last known doses of ZMapp, a drug that had earlier been given to two infected Americans and a Spaniard. The Americans are also improving, but the Spaniard died.

"The medical professionals have informed the Liberian information ministry their progress is 'remarkable,"' the ministry said in a statement, adding that the patients are showing "very positive signs of recovery."

Experts have said it is unclear if ZMapp, which had never before been tested in humans, is effective. Even if it is, the California-based maker has said more supplies won't be available for months.

In the meantime, experts say the best way to stop the spread of Ebola in West Africa is to identify the sick, isolate them from the healthy and monitor everyone with whom they have been in contact.

More than 1,200 people have died of Ebola in Guinea, Liberia, Sierra Leone and Nigeria in the current outbreak, and more than 2,200 have been sickened, the WHO said.

Local staff and healthcare workers for Doctors Without Borders in LIberia wear Ebola protection equipment. Cases in West Africa's Ebola outbreak this year have risen to 2,240. (MSF/Associated Press)

Authorities have struggled to treat and isolate the sick, in part because of widespread fear that treatment centres are places where people go to die. Many sick people have hidden in their homes, relatives have sometimes taken their loved ones away from health centres, and mobs have occasionally attacked health workers.

Liberia's government imposed a night curfew to prevent the spread of the Ebola virus, state radio said Tuesday.

On Saturday, residents of the West Point slum in Liberia's capital of Monrovia attacked a centre where people were being monitored for Ebola. The raid was triggered by fears that people with the disease were being brought there from all over the country, the Information Ministry said Tuesday.

During the raid, dozens of people waiting to be screened for Ebola fled the centre. Looters made off with items, including bloody sheets and mattresses that could spread the infection.

All the patients who fled are now being screened at a hospital in Monrovia, and those who tested positive are being treated, the ministry said. It was unclear how many of the 37 who fled were confirmed with Ebola. In addition, residents of the slum have agreed to return any stolen items, officials said.

Liberian authorities are searching for a pastor who ran away from a different Ebola treatment centre outside Monrovia. State radio asked the public to look out for the preacher but did not say whether he had tested positive for Ebola.

The WHO said it is seeing some encouraging signs in other parts of West Africa. In Guinea, people from villages that had previously rejected outside help were beginning to seek medical care, according to a WHO statement. The statement said the situation is "less alarming" in Guinea than in Liberia and Sierra Leone.

Although the outbreak began in Guinea, Liberia has now recorded the highest number of deaths and Sierra Leone the most cases.

The WHO also said there is "cautious optimism" that the spread of the virus in Nigeria can be stopped. So far, all recorded cases have been linked to one man, who flew from Liberia to Nigeria while infected.

"The outbreak is not under control," the WHO cautioned. "As recent experience shows, progress is fragile, with a real risk that the outbreak could experience another flare-up."

To try to stem the spread of Ebola, officials have imposed quarantines and travel restrictions on the sick and those in contact with them, sometimes shutting off entire villages and counties.

Those restrictions are limiting access to food and other necessities, said the WHO. The UN World Food Program has said that it is preparing to deliver food to 1 million people over the next three months.