Up to 100 people may have had direct or indirect contact with the first person to be diagnosed with the Ebola virus in the United States, and four of his relatives have been quarantined in an apartment in Dallas, health officials said.

Dallas County officials said 12 to 18 people had direct contact with the patient, who had recently flown in from Liberia, and they in turn had contact with scores of others. Liberia named him as Thomas Eric Duncan, a resident of its capital Monrovia.

The worst outbreak of the contagious, haemorrhagic fever on record has killed at least 3,300 people in Guinea, Sierra Leone and Liberia, overwhelming weak health systems and crippling fragile economies and threatening to cause hunger .

Liberia has recorded nearly 2,000 of those deaths and aid agencies say they need hundreds of beds for patients.

The case in the United States has heightened concerns that Ebola could spread globally and could raise further questions about travel restrictions from the affected countries.

US Secretary of Health and Human Services Sylvia Burwell and National Institute of Allergy and Infectious Diseases at the National Institutes of Health Director Dr Anthony Fauci held a briefing on the Ebola crisis from the White House in Washington on Friday.

United States Africa Command (Africom) commander, US Army General David Rodriguez, US Agency for International Development (Usaid) administrator Raj Shah were also present.

Liberian authorities on Thursday said they could prosecute Duncan if he returned because he had filled out a form before flying falsely stating he had not come into contact with an Ebola case, when he had actually helped a neighbour with the disease days earlier.

Liberia's President Ellen Johnson Sirleaf told Canadian Broadcasting Corp. she was angry with Duncan for what he had done, especially given how much the United States was doing to help tackle the crisis.

"One of our compatriots didn't take due care, and so, he's gone there and in a way put some Americans in a state of fear, and put them at some risk, and so I feel very saddened by that and very angry with him," she said.

"The fact that he knew (he might be a carrier) and he left the country is unpardonable, quite frankly," she said.

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

By Sunday, he needed an ambulance to return to the same hospital.

Police and armed security guards were keeping people about 100 yards away from the apartment on Thursday, with orange cones blocking the entrance and exit. Maintenance workers using high-pressure water were scrubbing the parking lot with bleach. The workers were not wearing any protective gear.

Officials said none of those thought to have had direct or indirect contact with the patient were showing symptoms of Ebola. The disease can cause fever, bleeding, vomiting and diarrhea and spreads through contact with bodily fluids.

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. Dallas County officials, said however, that more than 80 had direct or indirect contact.

"Out of an abundance of caution, we're starting with this very wide net, including people who have had even brief encounters with the patient or the patient's home," Texas health department spokeswoman Carrie Williams said.

The patient was in serious condition, Texas Health Presbyterian Hospital in Dallas said, no change from Wednesday. Four of his relatives were quarantined in an apartment in Dallas, added officials.

The director of US Centers for Disease Control and Prevention Thomas Frieden said he "remains confident" the United States can contain the virus there.

But the head of the International Monetary Fund, Christine Lagarde, on Thursday cited any expansion of the Ebola outbreak as one of several geopolitical risks to the global economy.

In one indication of its economic impact, the president of Emirates airlines said on Thursday that demand for flights to Africa from Asia has fallen due to concerns over Ebola.

Exxon Mobil has seen some of its oil and gas activities in Western Africa disrupted by Ebola including plans to drill offshore Liberia, the company's chief officer said.

"We had some drilling plans for some blocks offshore West Africa in Liberia," CEO Rex Tillerson told a news conference. "We are having to look at when it would be prudent to resume operations there because you do have to have shore-based support."

Exxon, which has operations in Nigeria and Liberia, is prohibiting employee travel to the countries directly affected by the disease and is taking precautionary measures related workers' families, the executive said.

A patient suffering from Ebola will arrive in the German city of Frankfurt for treatment in an isolation station in the city's University Hospital, a spokesman for the clinic said.

German media said the patient is a Belgian doctor who was working in Sierra Leone, which authorities would not confirm.-Reuters