WASHINGTON/MONROVIA, Aug. 7 -- A U.S. health official warned Thursday it would take at least three to six months to end the Ebola outbreak, which has killed nearly 1,000 in West Africa, and which has prompted a state of emergency in Liberia and Nigeria.

"It will be a long and hard fight," Director of the U.S. Centers for Disease Control and Prevention (CDC) Tom Frieden told a congressional hearing.

The director on Wednesday activated the level of the agency's response to the outbreak to its highest alert status.

Calling it "a crisis" and "unprecedented," he said at the current trend there will be more cases in this outbreak than in all previous recognized outbreaks of Ebola put together.

The outbreak, by far the largest in the nearly 40-year history of the disease, has infected over 1,700 people and killed nearly 1,000 so far this year in Guinea, Liberia, Nigeria and Sierra Leone, according to the World Health Organization (WHO).

GLOBAL EMERGENCY

Both Liberia and Nigeria declared a state of emergency on Wednesday over the Ebola outbreak.

The state of emergency allows Liberia's government to curtail civil rights and deploy troops and police to impose quarantines on badly affected communities.

The health care system in Liberia was collapsing, with hospitals closing, medical workers fleeing and people dying of common diseases because they are afraid to seek treatment, Liberia's Minister of Foreign Affairs Augustine Kpehe Ngafuan was cited by the Reuters as saying.

The worsening outbreak has prompted the United States to order the families of its diplomats in Liberia to leave and warn against non-essential travel to the West African nation.

A State Department statement said the U.S. staff would remain on active duty at the embassy and an additional staff was being sent to help the government tackle the outbreak of the deadly virus.

In eastern Sierra Leone, the worst-hit area of the country, military and the police have surrounded Kenema and Kailahun districts, preventing all vehicular traffic from leaving or entering the areas.

On Friday, the World Health Organization (WHO) declared that the Ebola outbreak in West Africa constitutes a public health risk, and met the conditions for a Public Health Emergency of International Concern.

After two tele-conferences of the emergency committee by the WHO's Director-General under the International Health Regulations, the committee advised that the Ebola outbreak in West Africa constitutes an "extraordinary event" and a public health risk to other states.

NEW DRUGS

While there are currently no licensed drugs for the deadly disease, officials from some Western African countries have called for the use of experimental drugs to be made available in their own countries.

An experimental drug called ZMapp has caught worldwide attention after two U.S. patients have shown signs of improvement after being treated with the medicine.

However, U.S. officials expressed their caution towards a wider application, citing that the drug has never been tested in people and there's no proof that it helped the two Americans.

The WHO, which is currently convening an emergency meeting in Geneva, will gather medical ethics experts next week to discuss whether to apply untested and unproven treatments in a wider scope.

While Zmapp is currently out of reach, another experimental drug to treat Ebola has possibly been given a "go-ahead."

On Thursday, Canadian drugmaker Tekmira Pharmaceuticals Corp. said its experimental drug TKM-Ebola has recently been eased safety restrictions by the U.S. Food and Drug Administration (FDA), a move that could clear the way for its future application.

The announcement, however, was not confirmed by the FDA, whose spokeswoman Stephanie Yao said FDA regulations bar the agency from disclosing information about experimental drugs.

Tekmira has a 149 million U.S. dollar contract with the U.S. government to develop the drug, which targets the genetic material of Ebola. But the FDA halted a small study of the injection in adults in July to request additional safety information.

Ebola, which spreads through body fluid or secretions such as faeces, urine, saliva and semen, can kill up to 90 percent of those infected. The fatality rate of the current epidemic is about 60 percent.