Christopher Stokes of Medecins Sans Frontieres (MSF) 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.
The disease has killed about 4,500 people so far, mostly in West Africa.
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.
Donors have given almost $400m (£250m) to UN agencies and aid organisations, following an appeal launched in September for $988m.
Separately, the UN is seeking $1bn for an Ebola trust fund, to provide a flexible source of back-up money to contain Ebola.
But UN Secretary General Ban Ki-moon on Thursday made another urgent appeal, saying the trust fund had received only $100,000 – from Colombia – though $20m has been pledged.
Meanwhile, the WHO has announced that Senegal is now officially free of Ebola, as it has gone 42 days without any sign of the virus.
There was one confirmed case of Ebola in the country, in late August, and the patient survived.
Mr Stokes, who leads MSF’s Ebola response, said promises from the international community were encouraging “but it is not having any significant impact on the epidemic and it won’t now for maybe another month or month and a half”.
“We’ve been calling for massive deployments for several months now and the deployments are always behind the curve.”
Another NGO, Action Aid, said the outbreak had to be tackled at source in West Africa.
Its head of humanitarian response, Mike Noyes, said in a statement: “There remains an urgent demand in countries like Liberia and Sierra Leone for more doctors, nurses, medical supplies and support for preventative measures.”
Calls for more aid have also been made in recent days by US President Barack Obama, UK PM David Cameron, and World Bank President Jim Yong Kim.
Separately, evidence has emerged suggesting there are concerns inside the World Health Organization that it failed to respond adequately to the outbreak earlier this year.
The Associated Press news agency says it has obtained an internal draft document in which WHO officials acknowledge failing to appreciate the seriousness of the situation as the number of cases grew.
“Nearly everyone involved in the outbreak response failed to see some fairly plain writing on the wall,” the document says, according to AP.
MSF warned last April that the spread of Ebola in West Africa was unprecedented and becoming uncontrollable – a position rejected at the time by the WHO.