DALLAS (REUTERS) - The Liberian government could prosecute the first person diagnosed in the United States for Ebola for denying that he had contact with someone who was eventually diagnosed with the sickness.

The head of Liberia's airport authority Binyah Kesselly said Thomas Eric Duncan failed to declare that he helped a pregnant neighbour after she fell critically ill on Sept 15. She later died of Ebola, Kesselly said.

Days later, Duncan flew to Texas via Brussels and Washington.

At Liberia's airport, he was asked in a questionnaire whether he had come in contact with any Ebola victim or was showing any symptoms.

"To all of these questions, Mr Duncan answered 'no,'"Kesselly said.

After arriving in Texas, Duncan stayed in an apartment in the north-eastern part of the city for about a week before going to a Dallas hospital.

Four people close to Duncan are being quarantined in the Dallas apartment, where sheets and other items used by Duncan were sealed in plastic bags, as health officials widened their search for others who had direct or indirect contact with him.

Ebola can cause fever, bleeding, vomiting and diarrhoea and spreads through contact with bodily fluids such as blood or saliva.

Meantime in Liberia, an American freelance television cameraman working for NBC News in Liberia has contracted Ebola, the fifth US citizen known to be infected with the deadly virus that has killed at least 3,300 people in the current outbreak in West Africa.

The 33-year-old man, whose name was not released, will be flown back to the United States for treatment, the network said on Thursday.

Immediately after beginning to feel ill and discovering that he was running a slight fever, the cameraman quarantined himself. He then went to a Doctors Without Borders treatment centre and 12 hours later, learned that he tested positive for Ebola.

The entire NBC crew will fly back to the United States on a private charter plane and will place themselves under quarantine for 21 days, the maximum incubation period for Ebola.

US health officials said they were confident they could prevent the spread of Ebola in the United States after Duncan's case was diagnosed this week on US soil.

Up to 100 people had direct or indirect contact with Duncan, and a handful were being monitored, said Dr Thomas Frieden, director of the US Centres for Disease Control and Prevention (CDC). None of those thought to have had contact with Duncan were showing symptoms of Ebola, Dallas County officials said at a news conference.

President Barack Obama called Dallas Mayor Mike Rawlings on Thursday and "pledged federal agencies will remain in close coordination and reiterated his confidence in America's doctors and national health infrastructure to handle this case safely and effectively," White House spokesman Eric Schultz said.

Officials have said the US healthcare system is well prepared to contain the hemorrhagic fever's spread by careful tracking of those who have had contact with Duncan, and employing appropriate care.

Dallas County officials said the problem was very localised."When I say local, I don't mean Dallas. I mean a very specific neighbourhood in the northeast part of Dallas," Dallas Mayor Rawlings told reporters.

Hospital sent patient away

Duncan initially sought treatment at Texas Health Presbyterian Hospital on the night of Sept 25 but was sent back to the apartment, with antibiotics, despite telling a nurse he had just been in Liberia.

By Sunday, he needed an ambulance to return to the same hospital after vomiting on the ground outside the apartment complex.

He was in serious condition on Thursday, no change from Wednesday, a hospital spokeswoman said.

Police and armed security guards were keeping people away from the apartment, with orange cones blocking the entrance and exit. Maintenance workers scrubbed the parking lot with high-pressure water and bleach.

Dr David Lakey, commissioner of the Texas Department of State Health Services, said the four people under quarantine did not have a fever and were healthy. Lakey said monitoring included fever checks twice a day.

At the apartment, "there is a law enforcement person there in case individuals leave," Lakey told reporters on a conference call. US officials initially described the number of people potentially exposed as a handful, and on Wednesday said it was up to 18. Then on Thursday, the Texas health department said there were about 100 potential contacts.

CNN reported that a Dallas woman who had a child with Duncan said he had sweated profusely in the bed they shared at her apartment. The woman, whom CNN identified only as "Louisa," is quarantined in the apartment with one of her children, who is 13, and two visiting nephews in their 20s. They were all in the home when Duncan began showing signs of illness, the report said.

The woman said she mentioned twice to hospital staff that he had come from Liberia.

Dr Amesh Adalja, an infectious disease physician at the University of Pittsburgh, said contact tracing is "bread-and-butter public health" and something health officials do regularly to track tuberculosis, measles and sexually transmitted diseases. Adalja said the most disturbing part of the US incident is that Duncan was sent home from the hospital with antibiotics.

"This really is something that shouldn't have happened," he said. "It just reinforces that taking a travel history has to be an essential part of taking care of patients."