UN health agency say outbreak is accelerating in the region while the death toll has now exceeded 1,500

The World Health Organisation (WHO) has warned the number of Ebola cases could rise to 20,000 as doctors in Liberia say the deadly virus is now spreading so rapidly they can no longer deal with the crisis.

The UN health agency said the outbreak is accelerating in west Africa, where the death toll has now reached 1,552, and it believes the numbers who have been hit by Ebola could be two to four times higher than the current 3,069 cases currently reported.

"[It] is a scale that I think has not ever been anticipated in terms of an Ebola outbreak," said Bruce Aylward, assistant director general of WHO.

He said the increase came from cities including the Liberian capital Monrovia, where a slum was quarantined last week, leading to food shortages and civil unrest.

"It's really just some urban areas that have outstripped the reporting capacity," he said. Up to now most efforts have concentrated on rural areas close to the Guinea border. His remarks come as Medecins sans Frontieres said it was struggling to cope with the caseload in Monrovia. MSF has just opened a new Ebola hospital in the Liberian capital and after one week it's already at capacity of 120 patients.

"The number of patients we are treating is unlike anything we've seen in previous outbreaks," said Lindis Hurum, MSF's emergency coordinator in Monrovia. "This is not an Ebola outbreak, it is a humanitarian emergency and it needs a full-scale humanitarian response."

The Ebola outbreak started in Guinea in March and is the 26th since 1976 when the virus was first identified, but is widely recognised to be out of control.

"It is simply unacceptable that, five months after the declaration of this outbreak, serious discussions are only now starting about international leadership and co-ordination," said MSF director of operations Brice de la Vigne.

MSF said the number of people seeking care at its new Monrovia centre is "growing faster than we can handle both in terms of the number of beds and the capacity of the staff". It said patients are coming from nearly every district of the city and healthcare workers were "struggling to screen new arrivals, care for admitted patients, safely remove dead bodies and transport them to the crematorium".

It said it is so overwhelmed it can no longer administer intravenous treatments. MSF has five field hospitals in west Africa, plugging a gap left by the fragile health care systems. It says most of the medical facilities in Monrovia have shut down over fears of the virus among staff, leaving many people with no healthcare at all. This is leading to fears of a secondary health crisis with expectant mothers and malaria patients now going untreated too.

Tom Dannatt is founder of British charity Street Child, which has 650 volunteers in two of the worst stricken countries, Sierra Leone and Liberia. He says the catastrophic spread was not to do with the strength of the virus but the lack of "anti-Ebola measures" being put in place. He also said a food aid programme to help feed those in quarantined areas in eastern Sierra Leone is completely inadequate.

"They are providing food for a standard World Food Programme five heads per household. But in Sierra Leone there are 12 to 15 people living in the majority of households," said Dannatt. He is offering to lend Street Child's workers to government task forces to help contain the disease and is launching an emergency appeal later this week.

Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases told the Guardian that containment could be achieved by "low tech public protection measures".

"What we have here is porous borders, poverty and big cities. It is the perfect storm. What we need is a massive influx of resources from WHO, from the US, from the UK," he said.

WHO said it is launching a new $489m (£294m) initiative to try to contain Ebola within six to nine months.

Aylward said it would require the assistance of 750 international workers and 12,000 national workers. He urged airlines, including British Airways, Air France and Gambia Bird, who have suspended flights to the affected countries, to restart services.