Liberia was declared free from Ebola on Saturday after 42 days without a new case, the medical charity Médecins Sans Frontières said, but it urged vigilance until the worst-ever recorded outbreak of the virus was extinguished in neighbouring Guinea and Sierra Leone.

A total of 11,005 people have died from Ebola in Liberia, neighbouring Guinea and Sierra Leone since the outbreak began in December 2013, according to the World Health Organization.

Nearly half of those deaths have been in Liberia, where the outbreak peaked between August and October, with hundreds of cases a week, sparking international alarm. The United States sent in hundreds of troops to help build treatment clinics in a country founded by freed U.S. slaves.

Helped by the visible U.S. military presence, President Ellen Johnson Sirleaf's government launched a national awareness campaign to stem the infectious disease, which is spread by physical contact with sick people.

MSF said that Liberia's completion of the WHO's benchmark for the end of an Ebola epidemic - 42 days without a new case, marking twice the maximum incubation period of the virus - should not lead to complacency.

"We can't take our foot off the gas until all three countries record 42 days with no cases," said Mariateresa Cacciapuoti, MSF's head of mission in Liberia.

She urged Liberia to step up cross-border surveillance to prevent Ebola slipping back into the country.

The UN Special Envoy on Ebola, David Nabarro, said this week that Liberian authorities had pledged to maintain heightened surveillance for at least a year after being declared Ebola-free on Saturday.

Nabarro suggested that, even though fewer than 20 new cases were reported in Guinea and Sierra Leone last week, it could take months to get to zero.

507 health workers killed by virus

International aid organisations were forced to step in as the Ebola outbreak ravaged the region's poorly equipped and understaffed healthcare systems.

MSF — which was highly critical of the slow response by the UN and western governments — opened the world's largest Ebola management centre in Monrovia, with a capacity of 400 beds.

According to the WHO, a total of 868 health workers have caught the virus in Guinea, Liberia, and Sierra Leone since the start of the outbreak, of whom 507 died.

International Medical Corps, a charity that ran two Ebola clinics in Liberia, appealed for international support in rebuilding the healthcare system there in the wake of the virus.

"Now is the time to build on the momentum we have generated to strengthen the Liberian health system ... and change attitudes to keep the people of Liberia safe long into the future," said Anouk Boschma, IMC's acting country director in Liberia.