The scale of the Ebola outbreak appears to be "vastly underestimated", the UN's health agency says, as the death toll from the disease reaches 1,069.

The World Health Organization said its staff had seen evidence that the numbers of reported cases and deaths do not reflect the scale of the crisis.

It said in a statement that "extraordinary measures" were needed.

The outbreak began in Guinea in February and has since spread to Liberia, Sierra Leone and Nigeria.

However, the WHO said the risk of transmission of Ebola during air travel remained low, as the disease is not airborne.

As a consequence, Kenya Airways has rejected pressure to suspend its flights to the Ebola-hit states of West Africa.

The WHO said the outbreak was expected to continue "for some time".

"Staff at the outbreak sites see evidence that the numbers of reported cases and deaths vastly underestimate the magnitude of the outbreak," its statement said.

"WHO is co-ordinating a massive scaling up of the international response."

Part of the challenge was the fact that the outbreak was in "settings characterised by extreme poverty, dysfunctional health systems, a severe shortage of doctors and rampant fear", the WHO added.

Two people have died in Nigeria after drinking a salt solution rumoured to prevent Ebola infection.

The BBC's Ishaq Khalid says text messages began circulating in Nigeria towards the end of last week recommending that people drink and bath in a salt solution as a way to stop getting the virus, for which there is no cure or vaccine.

Despite the health minister scotching the rumour, many people have been admitted into hospital after drinking salt water.

Ebola is transmitted by direct contact with the body fluids of a person who is infected.

Initial flu-like symptoms can lead to external haemorrhaging from areas such as eyes and gums, and internal bleeding which can lead to organ failure.

(BBC)