"The Ebola crisis could become the definitive humanitarian disaster of our generation," a spokesperson for the British-based charity said as it appealed for EU foreign ministers meeting in Luxembourg on Monday to do more.
Obama's warning about hysteria came a day after the World Bank said the battle against the disease was being lost and as the US president named an "Ebola czar" to co-ordinate Washington's response.
In Sierra Leone, Defence Minister Alfred Paolo Conteh was put in charge of the fight against the disease as the death toll there rose to 1,200.
In a statement, President Ernest Bai Koroma said the defence minister would "with immediate effect" head a new national Ebola response centre.
A global UN appeal for nearly US$1 billion (A$1.08 billion) to fight the spread of the disease has so far fallen short, but a spokesman told AFP more money was coming in daily.
Out of US$988 million requested a month ago, the UN said Saturday US$385.9 million had already been given by a slew of governments and agencies, with a further US$225.8 million promised.
"It has been encouraging to see the amount and the speed with which these amounts have been committed," said Jens Laerke, spokesman for the UN's humanitarian office (OCHA).
But the total was still some way off, Laerke said.
"Nobody's smiling in this crisis, so I'm not going to go out and clap my hands and say everything is going fine, because it's not," he told AFP.
As panic and Ebola scares spread worldwide, Obama called for patience and perspective.
"This is a serious disease, but we can't give in to hysteria or fear - because that only makes it harder to get people the accurate information they need. We have to be guided by the science," Obama said.
Friday saw a number of false alarms in the United States as fears grew, including at the Pentagon, where an entrance was closed after a woman vomited in a parking lot. US authorities later found no evidence that she had Ebola.
The United States - where a Liberian man died from Ebola on October 8 and two American nurses who treated him have tested positive - was not seeing an "outbreak" or "epidemic", Obama stressed.
More "isolated" cases in the country were possible, he conceded. "But we know how to wage this fight."
The US president played down the idea of a travel ban from west Africa.
"Trying to seal off an entire region of the world - if that were even possible - could actually make the situation worse."
Airports in several countries were taking passengers' temperatures in a bid to detect Ebola carriers, although experts have expressed doubts about the effectiveness of the checks.
France on Saturday started carrying out health checks on Air France passengers arriving from Guinea, where the epidemic began in December, while a cabin crew union called for a halt to flights from Conakry altogether.
The United States, Britain and Canada have already launched screenings at airports for passengers from Ebola-hit zones. The EU is reviewing the matter.
As of October 14, 4,555 people have died from Ebola out of a total of 9,216 cases registered in seven countries, the World Health Organisation said.
- AAP
