Image copyright Akron Public Schools Image caption Amber Vinson was not supposed to travel on an aeroplane, health officials said

US President Barack Obama has played down the chance of an Ebola outbreak in the US, saying the risk of Americans getting the virus was "extremely low".

He was speaking after a second US nurse became infected after treating a Liberian who died a week ago in Dallas.

Meanwhile, a UN chief has warned that West Africa faces a potential food crisis because of the Ebola crisis.

The disease has killed about 4,500 people so far, mostly in Liberia, Guinea and Sierra Leone.

In the US, health officials are seeking to trace 132 people who flew on a plane with Amber Vinson the day before she fell ill.

The US Centers for Disease Control and Prevention (CDC) said it wanted to interview the passengers on Frontier Airlines flight 1143 from Cleveland, Ohio, to Dallas, Texas on Monday.

An unnamed federal official has told US media that Ms Vinson, 29, called the CDC before boarding to report a temperature of 99.5F (37.5C) and to say she was getting on a plane.

She is the second nurse to become infected after treating Thomas Eric Duncan at the Texas Health Presbyterian Hospital.

Media playback is unsupported on your device Media caption President Obama: If Ebola rages "out of control" it will spread globally

More than 70 medics who may have come in contact with Mr Duncan at the hospital are being monitored for symptoms.

Mr Obama cancelled a political campaign trip to meet Cabinet officials involved in the Ebola response both in the US and in West Africa. He has also cleared his diary for Thursday, postponing other engagements so that he can remain in Washington and monitor the situation.

Willing volunteers

On Thursday, campaign group Avaaz said it had identified more than 2,000 international volunteers, including hundreds of doctors and nurses, who are willing to help fight the Ebola outbreak in West Africa.

Media playback is unsupported on your device Media caption Farmers in Sierra Leone have deserted their land during the Ebola outbreak

The group said it will give the names to aid agencies working in the field but they are unlikely to arrive in the region in the near future because of the training needed before they can be deployed.

Meanwhile, the head of a United Nations agency said on Wednesday that the West African states affected by the outbreak were facing a looming food crisis.

Kanayo Nwanze, the president of the UN's International Fund for Agriculture, said farmers in Sierra Leone, Guinea and Liberia had abandoned their crops because of fear of catching the disease.

'Aggressive monitoring in US'

Speaking after chairing a crisis meeting on Ebola, President Obama said it would be more difficult to prevent an outbreak in the US if the epidemic "rages out of control in West Africa".

Media playback is unsupported on your device Media caption Ms Vinson has been transferred to a specialist treatment unit in Atlanta

He promised a "much more aggressive" monitoring of Ebola cases in the US and reaffirmed plans to send a "Swat team" of experts to any hospital that reported an infection.

It is still unclear how Ms Vinson and the other infected nurse, Nina Pham, contracted the virus.

Both wore face shields, hazardous materials suits and protective footwear as they drew blood and dealt with Mr Duncan's body fluids.

However, a national nurse union said health workers had not been properly equipped while treating Mr Duncan.

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The Ebola fumble in Dallas

RoseAnn DeMoro, executive director of Nurses United, said staff treated the Liberian man for days without the necessary protective gear, and hazardous waste was allowed to accumulate.

The CDC has appointed a "site manager" at the Dallas hospital to standardise the protective equipment and supervise the method of putting it off and on.

Questions have also been raised about how Ms Vinson was allowed to board a flight, even though she was being monitored for signs of Ebola.

Media playback is unsupported on your device Media caption Dr Richard Carmona: "If you forget to do something in that chain of protection it could be fatal"

When she returned from Ohio on Monday evening, she was not showing symptoms of the disease, the crew has told CDC investigators.

Health experts say people who are not showing symptoms are not contagious.

Ms Vinson has now been transferred to Emory University hospital in Atlanta, a specialist unit which oversaw the recovery of two US aid workers who had caught the disease in Africa.

Mr Duncan, who was the first person to be diagnosed in the US with Ebola, started showing symptoms of the disease just days after he arrived in Texas from Liberia.

The 48 people, including his girlfriend, who were in contact with him before he was admitted to hospital are nearing the end of the window in which they could develop an Ebola infection.

Ebola patients treated outside West Africa*

*In all but three cases the patient was infected with Ebola while in West Africa. Infection outside Africa has been restricted to health workers in Madrid and in Dallas. DR Congo has also reported a separate outbreak of an unrelated strain of Ebola.

How not to catch Ebola:

Avoid direct contact with sick patients as the virus is spread through contaminated body fluids

Wear goggles to protect eyes

Clothing and clinical waste should be incinerated and any medical equipment that needs to be kept should be decontaminated

People who recover from Ebola should abstain from sex or use condoms for three months

Ebola basics: What you need to know

How Ebola attacks

Ebola: Mapping the outbreak