LONDON (AFP) ― Aid agency Oxfam said Ebola could become the “definitive humanitarian disaster of our generation,” as U.S. President Barack Obama urged against “hysteria” in the face of the growing crisis.

Oxfam, which works in the two worst-hit countries ― Liberia and Sierra Leone ― on Saturday called for more troops, funding and medical staff to be sent to tackle the west African epicenter of the epidemic.

Chief executive Mark Goldring warned that the world was “in the eye of a storm.”

“We cannot allow Ebola to immobilize us in fear, but ... countries that have failed to commit troops, doctors and enough funding are in danger of costing lives,” he said.

The worst-ever outbreak of the deadly virus has so far killed more than 4,500 people, mainly in Guinea, Liberia and Sierra Leone, but isolated cases have now begun to appear in Europe and the United States.

“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 U.S. president named an “Ebola czar” to coordinate Washington’s response.

In Sierra Leone, Defense 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 defense minister would “with immediate effect” head a new national Ebola response center.

A global U.N. appeal for nearly $1 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 $988 million requested a month ago, the U.N. said Saturday $385.9 million had already been given by a slew of governments and agencies, with a further $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 U.N.’s humanitarian office.

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. U.S. authorities later found no evidence that she had contracted Ebola.

Meanwhile, U.S. media reported on overzealous action taken by some worried communities, including a group of Mississippi parents who pulled their kids from school because the principal had recently travelled to Zambia ― a southern African country far from the Ebola crisis in west Africa.

The United States ― where a Liberian man died from Ebola on Oct. 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 U.S. 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.”

Obama’s call for calm was in stark contrast to World Bank chief Jim Yong Kim, who warned Friday that “we are losing the battle.”

He blamed a lack of international solidarity in efforts to stem the epidemic.