A patient has tested positive for Ebola in Liberia's capital, deflating hopes that the West African nation had beaten the disease after weeks with no new cases, officials said.

Liberia has seen the most deaths in the West African Ebola outbreak, which has killed more than 10,000 people. But since it discharged its last case on March 5, it was counting down the 42 days countries must wait before being declared Ebola-free.

But on Friday, officials confirmed that a new patient had tested positive.

"A woman has been confirmed as an Ebola patient... This is a new case after we have gone more than 27 days without a single case. It is a setback," government spokesman Lewis Brown told the AFP news agency.

The woman went to the emergency room of Monrovia's Redemption Hospital on Thursday night, according to Elizabeth Hamann of the International Rescue Committee, which is helping the hospital reopen amid the outbreak.

She was identified as a suspected Ebola case and put in the hospital's isolation unit while awaiting test results. She is now at a treatment centre.

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In a worrying sign, it is not clear where the woman became infected. She does not seem to be linked to any of the people on an Ebola contacts list, and she has said that she did not travel to an infected country, officials said.

Authorities are considering the possibility that she had a visitor from outside Liberia who infected her or that she had sex with a survivor.

The Ebola virus can be found in the semen of survivors for up to three months, and health authorities recommend that survivors abstain from sex during that period.

An emergency meeting will be held Saturday to discuss the case.

Since the outbreak began in December 2013, 24,753 people in nine countries have been infected with the virus, and 10,236 of them have died, according to the latest figures.

All but 15 of those deaths have occurred in Liberia and its neighbours, Sierra Leone and Guinea.

Source: Agencies