Image copyright Reuters Image caption Barack Obama criticised foreign governments for not stepping up their response to the West Africa outbreak

The US is planning measures to screen incoming air passengers for Ebola, President Barack Obama has said.

The likelihood of an Ebola outbreak in the US is "extremely low", Mr Obama said, but "we don't have a lot of margin of error".

More than 3,400 people have died in West Africa in the world's deadliest outbreak of the viral disease.

Mr Obama's comments came six days after a Liberian man became the first case of Ebola diagnosed on American soil.

Thomas Eric Duncan, who contracted the disease in Liberia, is in a critical condition in a hospital isolation unit in Dallas.

Meanwhile, on Monday a plane carrying American journalist Ashoka Mukpo, who contracted Ebola while working in Liberia, landed in Nebraska, where he will undergo treatment for the deadly disease.

His parents said at a news conference he was looking strong and was "enormously relieved" to be in the US.

Media playback is unsupported on your device Media caption Mukpo, the American journalist who contracted Ebola, arrives in Nebraska

Mr Obama, who spoke after a White House briefing with US health officials, said the US government would push to ensure doctors and other medical professionals responded appropriately if they came in contact with a patient with Ebola-like symptoms.

"We're also going to be working on protocols to do additional passenger screening both at the source and here in the United States," he said.

The US president also criticised foreign governments for not acting "as aggressively as they need to" against the outbreak.

"Countries that think that they can sit on the sidelines and just let the United States do it, that will result in a less effective response, a less speedy response, and that means that people die.

"And it also means that the potential spread of the disease beyond these areas in West Africa becomes more imminent," Mr Obama said.

Image copyright Reuters Image caption A patient is disinfected in Monrovia, Liberia

Passengers leaving affected countries already have their temperatures checked, but people do not become infectious until they display symptoms.

Mr Duncan was screened for symptoms when he left Liberia but appeared healthy. He did not develop symptoms of Ebola until four days later, when he was in the US.

He is now in a critical condition in hospital and receiving an experimental drug treatment.

Media playback is unsupported on your device Media caption Mukpo's mother Diana Mukpo: "He's enormously relieved to be here"

Ten people who came into direct contact with him are being closely monitored but none have yet displayed any Ebola symptoms.

Dr Tom Frieden, director of the Centers for Disease Control and Prevention, has ruled out banning flights to the US from countries suffering the outbreak, arguing the isolation would only worsen the outbreak within Africa and would deny those countries crucial aid.

On Monday the first case of contagion outside Africa was confirmed in Spain, where a nurse who treated an Ebola victim in Madrid contracted the virus herself.

Image copyright Getty Images Image caption Liberians in New York held a fundraising concert

Dr Frieden has said he did not believe Ebola would spread in the US.

"We can stop it in its tracks here, which we are doing," he said.

A national survey by the Pew Research Center suggested most Americans trust the government to prevent a major outbreak - 20% have a "great deal" of confidence, while another 38% said they have a "fair amount" of confidence.