US President Barack Obama played down chance of ebola outbreak risk in the US

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.

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.

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".

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.