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.
Meanwhile, the international ratings agency Moody's says the Ebola outbreak - the world's deadliest so far - may have significant economic ramifications on the affected countries because commercial and transport disruptions are expected to last at least another month.
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.
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