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The US is planning measures to screen incoming air passengers for Ebola, according to president Barack Obama.

The likelihood of an outbreak of the deadly virus in the US is “extremely low”, 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.

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

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

Meanwhile, an aircraft carrying American journalist Ashoka Mukpo, who contracted Ebola while working in Liberia, landed yesterday in Nebraska, where he will undergo treatment for the deadly disease.

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.

Obama also criticised foreign governments for not acting “as aggressively as they need to” against the outbreak, the BBC reported.

“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,” he said.