An internal document said those involved "failed to see some fairly plain writing on the wall", according to the Associated Press.
Separately, sources close to the WHO told Bloomberg of multiple failures in the outbreak's early stages.
In the worst affected countries - Liberia, Guinea and Sierra Leone - the Ebola virus has now killed 4,546 people with cases of infection numbering 9,191, according to the latest WHO figures.
The reports have brought into focus the way the WHO dealt with the outbreak in the months after it received the first reports of Ebola cases in Guinea in March.
Medical charity Medecins Sans Frontieres (MSF) warned in April that the outbreak was out of control - something disputed by the WHO at the time.
"Nearly everyone involved in the outbreak response failed to see some fairly plain writing on the wall," the document obtained by AP says.
The draft report - a timeline of the outbreak - also reportedly says that experts should have realised that traditional methods of containing infectious disease would not work in a region with porous borders and poor heath systems.
Among the problems cited in the information obtained by AP and Bloomberg are:
A failure of WHO experts in the field to send reports to WHO headquarters in Geneva
Bureaucratic hurdles preventing $500,000 reaching the response effort in Guinea
Doctors unable to gain access because visas had not been obtained
Responding to the allegations, the WHO's head of global response and alert, Isabelle Nuttall, told the BBC: "Time will come for investigation. Right now we have to focus on the response."
|Head of global response and alert at the WHO, Isabelle Nuttall|
"When we scaled up, the beginning of the outbreak was very comparable to what we had seen elsewhere in Africa," she said. "And then, by June, it became something different.
"We indicated that this outbreak was different. I'm afraid we probably didn't say it loud enough for the world to understand what we were saying and for all the international community to be mobilised."
Earlier, WHO Director General Margaret Chan told Bloomberg that she was "not fully informed of the evolution of the outbreak" and the response might not have matched the "scale" and "complexity" of the spread.
|A medical worker dons protective gear before entering |
an Ebola treatment centre in Freetown, Sierra Leone
The combination of a leaked internal document and frank comments from the WHO's director general signal growing concerns about the effectiveness of the agency's efforts against Ebola.
Back in April, MSF described the outbreak in West Africa as unprecedented, warning that it risked spiralling out of control.
The WHO responded that it had seen only sporadic cases in a limited geographic area. It was not until August that the organisation suggested international reaction to Ebola may have been too slow.
Perhaps WHO officials feared accusations of overreacting: in 2009 the organisation swiftly declared a global pandemic of swine flu, advising countries to spend billions on treatments and vaccines against a virus which caused far fewer deaths than regular seasonal flu.
There are allegations too that the WHO's regional office in Africa may be part of the problem, that its staff failed to properly monitor West Africa's Ebola outbreak. We now know it began in December, but the first cases were not notified until March.
Earlier, MSF said international pledges of deployments and aid for Africa's Ebola-hit regions had not yet had any impact on the epidemic.
MSF's Christopher Stokes said the disease was still out of control.
He said it was "ridiculous" that volunteers working for his charity were bearing the brunt of care in the worst-affected countries.
Mr Stokes, who leads MSF's Ebola response, said international efforts would not have any effect for another month and a half.
MSF runs about 700 out of the 1,000 beds available in treatment facilities Liberia, Sierra Leone and Guinea.
The BBC's Mark Doyle, at the UN Ebola logistics base in Ghana, says it is generally agreed that at least three times that number are needed.
Responding to MSF's criticisms, David Nabarro, the UN system co-ordinator for Ebola, told the BBC that he had seen a big increase in the international response over the past two months.
"I am absolutely certain that when we look at the history, that this effort that has been put in place will have been shown to have had an impact, though I will accept that we probably won't see a reduction in the outbreak curve until the end of the year."
|Family suffering from Ebola waiting for treatment|
The WHO has announced that Senegal is now officially free of Ebola, as it has gone 42 days without any sign of the virus.
President Obama named Ron Klain - former chief of staff to Vice President Joe Biden - as Ebola "tsar" in charge of combating the virus in the US
Five East African countries - Kenya, Uganda, Tanzania, Rwanda and Burundi - are to send 600 personnel to help in the worst affected nations
How not to catch Ebola:
Avoid direct contact with sick patients as the virus is spread through contaminated body fluids
Wear goggles to protect eyes
Clothing and clinical waste should be incinerated and any medical equipment that needs to be kept should be decontaminated
People who recover from Ebola should abstain from sex or use condoms for three months