Media playback is unsupported on your device Media caption A fast response team has now been set up to deal with Ebola cases in the US, as Alastair Leithead reports

A major outbreak of Ebola in the US and elsewhere in the West is unlikely given the strong health systems, the World Health Organization (WHO) says.

US President Barack Obama also said the risk of Americans getting the virus was "extremely low", although he ordered a "much more aggressive response".

The US is investigating how a nurse infected when treating a victim in Texas was allowed to travel on a plane.

Officials are trying to trace the 132 people who flew with Amber Vinson.

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

EU health ministers are meeting in Brussels to discuss the crisis, including increased screening of travellers and the possibility of sending more troops to West Africa to help contain the virus.

Meanwhile, US federal health officials will appear before a congressional committee on Thursday to answer questions about their handling of the crisis.

'Very low'

Christopher Dye, WHO director of strategy, said the introduction of Ebola into the US or other countries in Western Europe was a matter "for very serious concern".

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

"The possibility that once an infection has been introduced that it spreads elsewhere, is something that everybody is going to be concerned about," he said.

But he added: "We're confident that in North America and Western Europe where health systems are very strong, that we're unlikely to see a major outbreak in any of those places."

Earlier, President Obama said the likelihood of a widespread Ebola outbreak was "very, very low".

New US Ebola control measures

A "site manager" will supervise how workers at the Texas Health Presbyterian Hospital put on and take off protective clothing

Two nurses from Emory University Hospital in Atlanta will offer "enhanced training" in Dallas

An immediate response team will travel to the site of any future Ebola diagnoses to hit the ground "within hours"

New guidelines for testing at hospitals throughout the US, with special emphasis on asking questions about travel history

Could it spread around US?

Confusing death figures

However, he promised a "much more aggressive" monitoring of Ebola cases in the US.

Britain, Canada and the US have introduced increased screening of travellers arriving at airports from West Africa.

France said on Wednesday that it would begin checks from Saturday on passengers arriving at Paris Charles de Gaulle airport from the Guinean capital, Conakry.

Nurse allowed to travel

US health officials are facing new questions about the response to Ebola infections in Texas.

Thomas Eric Duncan was the first person to be diagnosed in the US with Ebola after he flew in from Liberia. He was treated at the Texas Health Presbyterian Hospital but later died.

Image copyright EPA Image caption The disease has killed about 4,500 people so far, mostly in Liberia, Guinea and Sierra Leone

Image copyright PA Image caption The UK's 22 Field Hospital prepares to fly out to Sierra Leone

In testimony prepared for Thursday's congressional hearing, Daniel Varga, the chief clinical officer for Texas Health Resources, apologises for mistakes made in treating Mr Duncan.

"We did not correctly diagnose his symptoms as those of Ebola. We are deeply sorry," he says.

Two nurses, Nina Pham and Amber Vinson, contracted Ebola after treating Mr Duncan.

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

US media reaction

The New York Times says the appearance of the latest Ebola patient "provided more signs of concern about federal officials' ability to control the spread of the disease... and indications that the issue was becoming politicised"

A report by USA Today says health officials now believe Ebola patients should be treated at four specially designed US clinics rather than at hospitals around the country

Ebola is becoming "the next great American panic", writes Chico Harlan in the Washington Post

The Christian Science Monitor looks at how much large donations, such as the $25m (£16m) given by Facebook chief executive Mark Zuckerberg and his wife, Priscilla Chan, can help fight the spread of the disease

Ms Vinson later contacted the US Centers for Disease Control and Prevention (CDC) to inform it she was travelling on a plane on Monday - Frontier Airlines Flight 1143 from Cleveland to Dallas-Fort Worth.

She reported a temperature of 37.5C (99.5F) and CDC director Thomas Frieden said she should not have travelled on a commercial flight.

However, another health official told the New York Times later that Ms Vinson was not prevented from flying because the temperature was mildly elevated and was in a category not covered by the CDC.

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

Media playback is unsupported on your device Media caption The BBC's Alastair Leithead on how to wear a bio-hazard suit

"I don't think we actually said she could fly, but they didn't tell her she couldn't fly," the official told the Times. "She called us... I really think this one is on us."

Officials are trying to trace all 132 passengers but insist that as Ms Vinson did not have a fever, the risk to "any around that individual on the plane would have been extremely low".

Ms Vinson has now been transferred to hospital in Atlanta.

In other developments on Thursday:

An Air France passenger with a high fever, reportedly from Nigeria, is to be examined in hospital for Ebola symptoms after arriving in Madrid from Paris

Sierra Leone says the last district not to have registered any Ebola cases - Koinadugu in the far north - has now reported two infections

UN High Commissioner for Human Rights Zeid Raad al-Hussein says his agency is drawing up guidelines so that Ebola-hit nations do not violate human rights with quarantine rules

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