Media playback is unsupported on your device Media caption Liberians in Staten Island, New York, share their fears

The head of the US infectious disease agency has said he is "confident" the US will not suffer a large outbreak of the deadly virus Ebola, currently devastating countries in West Africa.

Dr Tom Frieden told a Congressional panel the US could expect isolated cases but well-prepared hospitals could prevent a widespread epidemic.

More than 900 have died in Africa.

Meanwhile, a New York man who had travelled to the region has tested negative for the virus.

Two Americans who are infected with Ebola were flown this week from Liberia to a hospital in Atlanta to receive treatment.

Ebola is one of the deadliest diseases known to humans, with a fatality rate in this outbreak of 50-60%.

Image copyright Reuters Image caption Dr Tom Frieden said the "best case scenario" would be an end to the outbreak in three to six months

It spreads through contact with the bodily fluids of Ebola patients showing symptoms.

In response to the African outbreak, the US Centers for Disease Control and Prevention (CDC) is now operating at its highest emergency response in order to free up resources.

"We do not view Ebola as a significant danger to the US because it is not transmitted easily, does not spread from people who are not ill, and because cultural norms that contribute to the spread of the disease in Africa - such as burial customs - are not a factor in the US," Dr Frieden of the CDC told Congress, according to his prepared remarks.

"We know how to stop Ebola with strict infection control practices which are already in widespread use in American hospitals."

Image copyright EPA Image caption Nurses in Liberia have been briefing the public

The CDC has sent 50 of its employees to the countries hardest-hit by the virus - Sierra Leone, Liberia, Guinea - as well as Nigeria.

"We have been able to stop every prior Ebola outbreak, and we will stop this one," Dr Frieden said.

"Ending this outbreak will take time, at least three to six months in a best case scenario, but this is very far from a best case scenario."

Image copyright Getty Images Image caption Dr Kent Brantly (right) was one of two Americans who received the new drugs

Meanwhile, the two infected Americans, aid workers Dr Kent Brantly and Nancy Writebol, are said to be improving after receiving an experimental drug called ZMapp, produced by a firm in San Diego. But it is unclear if the drug is responsible for their improving health.

Nigeria's health minister, Onyenbuchi Chukwu, said at a news conference that he had asked US health officials about access to the drug. Nigeria has seen seven confirmed cases.

On Wednesday, President Barack Obama said it was "premature" to decide whether or not to speed approval of the drug because there was not enough data.

"We're focusing on the public health approach right now because we know how to do that," Mr Obama said. "But I will continue to seek information about what we're learning with respect to these drugs going forward."

The treatment, tested only in animals, boosts the immune system's ability to fight off Ebola through antibodies made by lab animals exposed to elements of the virus.

The World Health Organization (WHO) has planned to convene a panel of medical ethicists next week to discuss the ramifications of using an untested drug and who should receive it given extremely limited supplies.

Some public health officials were wary of ramping up production of the drug.

"We don't even know if it works," said Dr Anthony Fauci of the National Institutes of Health.

Image copyright Getty Images Image caption Nigeria's Health Minister Onyebuchi Chukwu has asked for access to the experimental treatment

But Peter Piot, who co-discovered the virus in 1976, urged the drug be made more widely available.

"It is highly likely that if Ebola were now spreading in Western countries, public health authorities would give at-risk patients access to experimental drugs or vaccines," he said in a letter, along with two other Ebola experts, according to the LA Times newspaper.

The US Food and Drug Administration has separately given the US defence department an emergency authorisation to use an Ebola diagnostic test overseas.

It will be used in labs designated by the defence department to respond to the Ebola outbreak.