The World Health Organization said on Thursday that Ebola-ravaged Liberia was once again free of the deadly virus, prompting muted celebrations in the capital Monrovia.

The west African nation, where thousands died at the height of the epidemic last year, has already been declared Ebola-free once before, in May, only to see the fever resurface six weeks later.

"WHO declares Liberia free of Ebola virus transmission in the human population," the UN health agency said in a statement.

It hailed Liberia's "successful response" to the recent re-emergence of Ebola, when six people were infected, including two who died.

"Liberia's ability to effectively respond to the outbreak of Ebola virus disease is due to intensified vigilance and rapid response by the government and multiple partners," WHO said.

Liberia was long the hardest hit in the west African Ebola outbreak that began in December 2013 and which infected more than 28,000 people and claimed more than 11,000 lives mainly in Liberia, Guinea, and Sierra Leone.

More than 10,500 of those infections and 4,800 of the deaths occurred in Liberia.

Radio and television lunchtime bulletins carrying the development were greeted with muted celebration in Monrovia, a chaotic city of a million people where bodies piled up the streets at the outbreak's peak from August to December.

'Happy, and worried'

"I am happy, very happy to hear that pronouncement by the WHO but this is not the first time. So I am happy—and worried," said motorist Mary Grants, 56.

"I am worried because the experts told us that there will always be some re-occurrence of this virus, though it may not be like the first."

Ebola is spread among humans via the bodily fluids of recently deceased victims and carriers showing symptoms, including vomiting, diarrhoea and—in the worst cases—massive internal and external bleeding.

A country is considered free of human-to-human transmission once two 21-day incubation periods have passed since the last known case tested negative for a second time.

But experts warn that even after 42 days the danger is not over, considering that some Ebola cases are still surfacing in neighbouring Guinea and Sierra Leone.

The Ebola virus has been found lingering in the semen of male survivors many months after they test negative.

Francis Karteh, of Liberia's Ebola management department, warned that while the Ebola-free announcement was a cause for celebration, complacency could not be allowed to creep in and the fight against the virus was "not yet over".

"As long as there is one person with Ebola in our region Ebola is still a threat," he told AFP.

"The Ministry of Health and its partners will continue monitoring Liberia's borders and rebuilding the healthcare system to assure that Liberians remain safe."