Image copyright EPA Image caption Dr Richard Sacra was not treating Ebola patients

A US aid worker infected with Ebola in Liberia has arrived in Nebraska for treatment in an isolation unit.

Dr Rick Sacra, 51, a family doctor from Massachusetts, worked at the same hospital as the two other infected Americans, who are now recovering.

One of them, Nancy Writebol, said resources at the hospital were insufficient to protect workers.

The outbreak has killed more than 1,900 people and infected at least 3,500 in five West African countries.

There have been 3,500 confirmed or probable cases in Guinea, Sierra Leone and Liberia.

"The outbreaks are racing ahead of the control efforts in these countries," WHO chief Margaret Chan said.

Image copyright Getty Images Image caption Nancy Writebol was flown to the US for treatment after being infected with Ebola in Liberia

Nancy Writebol, who was flown to Atlanta for treatment last month, told the Associated Press news agency charities alone cannot handle the response to the disease.

Mrs Writebol and her husband David said about 250 employees at the hospital where she and Dr Sacra worked used thousands of disposable protective suits each week, but it was not enough to protect those doing the initial screenings for Ebola in an emergency room.

"We don't have enough personal protective safety equipment to adequately be able to safely diagnose if a patient has Ebola," David Writebol said, adding Ebola has "overwhelmed the supply chain".

Nurses in Liberia's largest hospital are on strike, refusing to return to work until they are issued with protective equipment.

Dr Sacra, 51, was not treating those infected with Ebola, but delivering babies and treating other patients. He had flown to Liberia shortly after Mrs Writebol and another aid worker were infected.

His plane landed early on Friday morning at a Nebraska Air Force base, arriving at hospital 40 minutes later by ambulance.

He will be treated at a 10-bed special isolation unit at the Nebraska Medical Center in Omaha, the largest of four such units in the US.

Image copyright AP Image caption A health worker sprays disinfectants on a man suspected of dying of Ebola in Liberia

Dr Sacra went into isolation after his temperature rose, but was well enough on Wednesday to be sending emails, a spokesperson for his aid organisation, SIM, said.

Doctors at the Nebraska hospital said he was able to board the plane to the US under his own power.

Ebola is only spread through close contact but there is no cure for the infection. The fatality rate for this outbreak has been almost 50%, according to the World Health Organization (WHO).

The director of the US Centers for Disease Control and Prevention (CDC) has called for the international community to step up its response, specifically a $490m (£298m) appeal from the WHO.

Media playback is unsupported on your device Media caption USAID Administrator Rajiv Shah tells the BBC the agency has pledged $100m to fight Ebola

The US Agency for International Development (USAID) which has directed the US response, has airlifted medical emergency supplies, including chlorine and gloves, to Liberia.

"The window of opportunity to stop Ebola from spreading widely throughout Africa and becoming a global threat for years to come is closing, but it is not yet closed," Dr Tom Frieden, who recently returned from West Africa, said in a statement

"If the world takes the immediate steps - which are direct requests from the frontlines of the outbreak and the presidents of each country - we can still turn this around."

His comments echo similar criticism by medical charity Medecins Sans Frontieres, who have said world leaders' response has been "lethally inadequate".