The UN's health agency said Thursday that the scale of the Ebola outbreak in West Africa has been vastly underestimated and "extraordinary measures" were needed to contain the disease.

As the official toll climbed to 1,069, according to World Health Organisation, the United States ordered the evacuation of diplomats' families from Sierra Leone, one of the three countries at the epicentre of the outbreak along with Liberia and Guinea.

The Geneva-based WHO said in a statement it was coordinating "a massive scaling up of the international response", in a bid to tackle the worst epidemic of haemorrhagic fever-causing virus since its discovery four decades ago.

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

"The outbreak is expected to continue for some time. WHO's operational response plan extends over the next several months," the organisation warned.

A serious outbreak in Lagos, where the epidemic claimed a fourth victim on Thursday, could severely disrupt the oil and gas industry in Nigeria if international companies are forced to evacuate staff and local operations are shut down, the Moody's rating agency warned.

Any "decline in production would quickly translate into economic and fiscal deterioration," said Matt Robinson, senior credit officer at Moody's.

Meanwhile, US President Barack Obama called President Ellen Johnson Sirleaf of Liberia and Sierra Leone's leader Ernest Bai Koromo.

The calls came as the US State Department ordered families of its diplomats in Sierra Leone to leave the country to avoid exposure.

"In his conversations with both leaders, the president underscored the commitment of the United States to work with Liberia, Sierra Leone, and other international partners to contain the outbreak and expressed his condolences for the lives lost," the White House said in a statement.

In Sierra Leone's parliament on Thursday, the country's chief medical officer, Dr Brima Kargbo, spoke of the difficulties health workers were facing in fighting the epidemic.

"We still have to break the chain of transmission to separate the infected from the uninfected," Kargbo said. But, he added: "There is a rejection among people of the existence of Ebola and hostility towards health workers."

The disease has taken its toll on those trying to help its victims.

Sierra Leone disclosed Thursday that 32 nurses died from Ebola while performing their duties between May 24 to August 13.

South Africa has stepped in to help the country by sending a mobile laboratory to be installed in the capital Freetown to ease the problem of having to send blood samples elsewhere for analysis, Sierra Leone's health ministry said.

- 'Need more centres' -

In Liberia, which has suffered more than 300 deaths, work began on Thursday to expand its Ebola treatment centre in the capital Monrovia -- one of only two centres in the country of 4.2 million.

"We need to increase the size of this place because more and more people arrive every day due to the awareness programme," Nathaniel Dovillie, head of the centre, told AFP.

The cost of tackling the virus threatens to exact a severe economic toll on the already impoverished west African nations at the epicentre of the outbreak -- Sierra Leone, Liberia and Guinea -- the Moody's rating agency warned.

"The outbreak risks having a direct financial effect on government budgets via increased health expenditures that could be significant," it said.

Liberia spent $12 million (nine million euros) tackling the Ebola outbreak between April and June, and looks set to spend much more in the coming weeks.

Increasingly draconian restrictions have been put in place across the region.

Guinea, where the outbreak has killed at least 377, declared a "health emergency" on Wednesday and ordered strict controls at border points and a ban on moving bodies "from one town to another until the end of the epidemic".

A number of airlines have cancelled flights in and out of West Africa. Gambia suspended all flights from Guinea, Liberia and Sierra Leone, according to a transport ministry document obtained by AFP.

Although the World Health Organisation confirmed that other African countries, including Kenya, were labelled at "high risk" due to their popular transport hubs, it also emphasised that "air travel, even from Ebola-affected countries, is low risk for Ebola transmission" because the virus is not airborne.

- Experimental treatments -

Canada's Health Minister Rona Ambrose said between 800 to 1,000 doses of a vaccine called VSV-EBOV, which has shown promise in animal research but never been tested on humans, would be distributed through the WHO.

Hard-hit nations were also anxiously awaiting a consignment of up to 1,000 doses of the barely tested drug ZMapp from the United States, which has raised hopes of saving hundreds infected with the disease.

There is currently no available cure or vaccine for Ebola, which the WHO has declared a global public health emergency. The body has said it is ethical to try largely untested treatments "in the special circumstances of this Ebola outbreak".

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