The deadliest Ebola outbreak ever is finally slowing in the worst-hit country of Liberia but it is still wreaking havoc in two neighbouring west African states amid warnings of thousands of unreported deaths.

The initially lacklustre global response to the crisis centred in Liberia and adjoining Sierra Leone and Guinea has gathered pace following repeated and impassioned appeals from top UN officials and world leaders.

But the good news from Liberia has been tempered by warnings that the global toll is likely vastly underestimated.

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The outbreak is officially thought to have claimed 4960 lives and infected 13,042 people, according to the latest data issued by the World Health Organization.

That could be the tip of the iceberg, an official at the UN health agency said.

“There are lots of missing deaths in this epidemic,” Christopher Dye, WHO’s strategy chief, told AFP, estimating that around 5000 fatalities could be missing from the count.

This assessment, he said, was based on the knowledge that the fatality rate in the epidemic stands at about 70 per cent.

Dye said the likely explanation was that many people were burying the dead in secret, possibly to avoid having authorities interfere with burial customs like washing and touching the deceased widely blamed for much of the transmission.

Sierra Leone’s President Ernest Bai Koroma pressed the point in a meeting this week with lawmakers well as tribal and religious chiefs.

“You must enforce the law and take out the sick,” he said, referring to a ban on traditional mourning rites with involve contact with corpses.

“This is time for action and you must stop the hypocrisy in the fight against Ebola,” added Koroma, whose country has recorded 1070 deaths from the disease and 4759 cases.

Even though the spread of the virus has slowed in Liberia, where 2697 people had died out of a total of 6525 cases, officials warned that this is no time for complacency.

“We cannot wait. This is a situation where we’re seeing progress but progress can be sporadic with this disease if we are not vigilant,” said Ertharin Cousin, the head of the UN’s World Food Programme this week while on a tour of west Africa.

More than 2000 children have been left orphans by the disease in Liberia alone, West Africa’s regional bloc ECOWAS said, urging international help to go beyond immediate medical care.

The United Nations said it has received just over half – $US572 million ($A618.88 million) of the $US988 million – the funds it is seeking to finance the fight against the worst outbreak of Ebola since the discovery of the viral disease in 1976.

US President Barack Obama is asking Congress for more than $US6.0 billion in emergency funding while Japan became the latest country this week to pledge extra aid, taking Tokyo’s contribution to a total of $US140 million.