Armed men claiming "there's no Ebola" in Liberia have raided a quarantine centre for the deadly disease in the capital Monrovia, prompting at least 20 patients infected with the deadly virus to flee, a witness said.

"They broke down the door and looted the place. The patients have all gone," said Rebecca Wesseh, who witnessed the attack.

Her report was confirmed by residents and the head of Health Workers Association of Liberia, George Williams.

Mr Williams said the unit housed 29 patients who were receiving preliminary treatment before being taken to hospital. It is unclear how many are now at large.

"They had all tested positive for Ebola," he said, adding that nine had died.

Ms Wesseh said she heard the assailants shouting that president Ellen Johnson Sirleaf "is broke", adding: "She wants money. There's no Ebola."

Ms Wesseh said the mostly young men armed with clubs broke into the isolation unit set up in a high school in a Monrovia suburb.

Nurses also fled the attack, she said.

Kenya closes borders to travellers from affected nations

The Ebola outbreak, the worst since the virus first appeared in 1976, has claimed 1,145 lives in five months, according to the UN World Health Organisation's latest figures: 413 in Liberia, 380 in Guinea, 348 in Sierra Leone and four in Nigeria.

Sorry, this video has expired Kenya closing borders to travellers from Guinea, Liberia and Sierra Leone ( Martin Cuddihy )

Medical charity Medecins Sans Frontieres has said the outbreak will take at least six months to bring under control.

Meanwhile Kenya has announced it is closing its borders to travellers from Guinea, Liberia and Sierra Leone in response to the deadly outbreak.

The World Health Organisation says Kenya is at "high risk" from Ebola because it is a major transport hub.

Returning Kenyan citizens and health workers will not be subject to the ban, but will still have to undergo strict tests and be quarantined if necessary.

Kenya Airways has since announced it will stop flights to Liberia and Sierra Leone when the ban comes in on Wednesday.

ABC/AFP