WASHINGTON (REUTERS) - US lawmakers on Thursday blasted the Obama administration's handling of Ebola on American soil, warning that public trust is waning after two nurses caring for a Liberian man with the disease contracted the virus.

"The trust and credibility of the administration and government are waning as the American public loses confidence each day, with demonstrated failures of the current strategy," said the chairman of a House of Representatives panel, Republican Timothy Murphy of Pennsylvania.

As pressure mounts for a US travel ban on countries in West Africa hardest hit by the Ebola epidemic, Republicans and Democrats spent hours at the House subcommittee hearing grilling the United States' top public health adviser.

US Centres for Disease Control and Prevention (CDC) director Thomas Frieden insisted that Ebola poses no major US health risk.

He also warned that a ban on travellers from Ebola-stricken countries in West Africa would make the disease harder to track and would pose a long-term danger to the US health system.

The worst Ebola outbreak on record has killed at least 4,493 people, mainly in Sierra Leone, Liberia and Guinea, and the number of cases are expected to soar to 20,000 by early November, according to the World Health Organisation.

"I will tell you, as the director of CDC, one of the things I fear about Ebola is that it could spread more widely in Africa. If this were to happen, it could become a threat to our health system and the health care we give for a long time to come," said Frieden.

He was testifying before a House oversight and investigations subcommittee that returned from an election-year recess to scrutinize the US response to the disease.

Doubts about US effectiveness in protecting Americans intensified after two Dallas hospital nurses who cared for Liberian patient Thomas Eric Duncan contracted the virus, and after news that one of the nurses, Amber Vinson, travelled aboard a commercial airliner while running a slight fever.

Duncan, who died on Oct 8, was initially sent away from Texas Health Presbyterian Hospital with antibiotics despite telling a nurse he had travelled from West Africa. He was taken back to the hospital two days later in an ambulance.

Dr Daniel Varga, chief clinical officer of the Texas hospital system that employs the nurses, apologised for mistakes in Duncan's treatment.

Murphy welcomed Varga's "statement of honesty" but pointed out that Frieden and four other administration officials at the witness table had not made similar admissions.

"I didn't hear that from any of you, and that troubles me. Because what has happened here is your protocol depends on everyone being honest 100 per cent of the time... People are not honest 100 per cent of the time," the lawmaker said.

Varga told the panel that there was no actual Ebola training for hospital staff before Duncan was admitted.

"It would be an understatement to say that the response to the first US-based patient with Ebola has been mismanaged, causing risk to scores of additional people," said Representative Diana DeGette, the subcommittee's top Democrat.

The response has fuelled questions about Frieden's credibility and stirred bipartisan calls for the appointment of an Ebola "czar" to oversee the government's response.

President Barack Obama has defended Frieden.

The CDC director presented an unapologetic face to the panel, which offered deference to Frieden even as lawmakers expressed misgivings about his repeated statements that Ebola poses no major health threat in the United States and can be defeated only by controlling the outbreak in West Africa.

"You have a difficult job," Representative Henry Waxman, a California Democrat, told Frieden.

"This is a fast-moving issue, and you're trying to explain things with people and educate them with limited information and partial authority."

Republican lawmakers pressed Frieden on the idea of protecting Americans at home by restricting travel for the 100 to 150 people who enter the United States daily from West Africa.

"You're right. It needs to be solved in Africa, but until it is, we should not be allowing these folks in, period," said Representative Fred Upton, Republican chairman of the House Energy and Commerce Committee.

"We would look at any proposal that would improve the safety of Americans," Frieden told lawmakers.

But he said a ban could prompt travellers from West Africa to hide their origin and rob authorities of the ability to track, identify and monitor those potentially exposed to Ebola.

"Right now, we know who's coming in. If we tried to eliminate travel, the possibility that some would travel over land and come from other places would mean that we won't be able to do multiple things," he said.

"We won't be able to check them for fever when they leave. We won't be able to check them for fever when they arrive. We won't be able... to take a detailed history to see if they were exposed."

The virus can also cause bleeding, vomiting and diarrhea, and spreads through contact with bodily fluids. It is not airborne.

Fears of infection from Vinson's Monday flight from Cleveland, Ohio, to Dallas, a day before she was diagnosed with Ebola, led to school closings in Ohio and Texas and caused airline stock prices to fall on Wall Street.