MONROVIA, Liberia - Liberia's president declared a curfew and ordered security forces to quarantine a slum home to at least 50,000 people late Tuesday as the West African country battled to stop the spread of Ebola in the capital.

The measures came as authorities said that three health workers in the country who received an experimental drug for the disease are showing signs of recovery, though medical experts caution it is not certain if the drug is effective.

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Authorities here have struggled to treat and isolate the sick, in part because of widespread fear that treatment centers 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 centers, and mobs have occasionally attacked health workers.

Liberian President Ellen Johnson Sirleaf announced late Tuesday that a curfew is going into place from 9 p.m. to 6 a.m. Security forces also will be ensuring no one goes in or out of West Point, a slum in the capital where angry residents attacked an Ebola observation center over the weekend.

"We have been unable to control the spread due to continued denials, cultural burying practices, disregard for the advice of health workers and disrespect for the warnings by the government," she said. "As a result and due to the large population concentration the disease has spread widely in Monrovia and environs."

"May God bless us all and save the state," she later added.

Saturday's attack on the observation center in West Point was triggered by fears that people with the disease were being brought there from all over the country, the Information Ministry said Tuesday. Dozens of people waiting to be screened for Ebola fled the center during the chaos. Looters made off with items, including bloody sheets and mattresses that could further spread the virus.

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.

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

Three Liberians are currently 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.

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. Late Tuesday, health authorities there announced a fifth Ebola death _ a doctor who had treated a man who flew to Nigeria from Liberia while infected. So far, all recorded cases have been linked to that man.

"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.