WASHINGTON -- Health workers on the front line of the Ebola crisis say the need for urgent help isn't letting up, as the U.S. Congress begins considering President Barack Obama's $6.2 billion emergency aid request to fight the disease.

Despite reports that the number of infections is slowing in some parts of West Africa, cases still are rising in other areas -- and aid organizations say thousands of health care workers are needed to treat Ebola over the next few months.

"We're not yet at a point where we can have confidence that we're turning the corner, even in Liberia," said Andy Gleadle of the International Medical Corps, which is running a treatment centre in Liberia and plans to open another in that country and two more in Sierra Leone.

Even with increasing global attention to the epidemic, it takes time to train new health workers, build field hospitals, and buy protective equipment for doctors and nurses.

"Let's say President Obama gives us another $5 million tomorrow morning -- which would be very nice, thank you -- but it takes weeks to absorb that funding and implement it on the ground," added Gleadle, who is responsible for the charity's response in Sierra Leone.

On Wednesday, the Senate Appropriations Committee is set to question Obama administration officials about the U.S. response to the Ebola outbreak as it begins evaluating the emergency aid request. It includes $4.64 billion in immediate spending to fight the epidemic in West Africa, shore up U.S. preparedness, and speed the development and testing of Ebola vaccines and treatments.

More than $1.5 billion would be for a contingency fund to deal with any unexpected developments.

But money doesn't solve everything, said Benoit Carpentier, a spokesman with the International Federation of the Red Cross, whose teams are carrying out safe burials and running a treatment centre. Time might be the scarcest resource now, he said: Time to reach remote villages and explain how the virus is transmitted, time to persuade people to change their behaviours so they don't spread the disease, time to track down people who have come in contact with the sick. It's difficult for health workers and others to keep up.

"The situation does change quite dramatically from one day to the next, one week to the next," Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, told The Associated Press.

Cases continue to pop up in new areas across the three hard-hit West African countries of Liberia, Sierra Leone and Guinea, he said.

"The challenges are really daunting, and one of the critical needs is for speed and flexibility" as those clusters emerge, Frieden said.

The hearing comes even as Ebola is fading from U.S. headlines.

The last Ebola patient being treated in the U.S. -- a doctor who was diagnosed after returning from a volunteer stint fighting the virus in Guinea -- was released from a New York City hospital Tuesday. The doctor's movements around New York before he became sick touched off a debate about whether health care workers like him should be automatically quarantined upon their return home. Carpentier warned that the stigma surrounding health care workers, both internationally and in the affected countries, is a growing problem.

That "doesn't mean we're out of the woods," Frieden cautioned. Until the epidemic is ended in West Africa, "there is still the real possibility that other people with Ebola will be diagnosed in the U.S."