Thomas Eric Duncan, right, with a friend at a wedding in Ghana. Duncan has been kept in isolation at a hospital since Sunday / AP

Authorities say Thomas Eric Duncan took a pregnant woman with Ebola to a clinic days before boarding flight

Liberian authorities have announced plans to prosecute the first man diagnosed with Ebola on US soil who they claim lied about not having any contact with an infected person before boarding a plane to America.

Thomas Eric Duncan is in a serious but stable condition at the Texas Health Presbyterian Hospital in Dallas after being admitted on Sunday. US health officials say the Liberian national, who was in Texas visiting family, may have come into direct or indirect contact with up to 100 adults and children before being hospitalised.

Their announcement comes as an American cameraman working for NBC was diagnosed with Ebola in Liberia and is due to be flown to the US for treatment.

Authorities say Mr Duncan was asked to fill in a questionnaire about his health and activities prior to boarding a flight to Dallas in an airport screening.

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Among the questions asked on the form, which has been obtained by The Associated Press, one asked whether Mr Duncan had cared for an Ebola patient or touched the body of anyone who had died in an area affected by Ebola. He answered no to all the questions.

Neighbours say Mr Duncan had days earlier helped carry to a taxi a pregnant woman who later died of Ebola. Her illness at the time was believed to be pregnancy-related.

At the time Mr Duncan left, it is not clear if he knew of the woman's diagnosis. Officials have said he was showing no symptoms when he boarded the plane and was therefore not contagious. Ebola can only be spread through the bodily fluids of people showing signs of the disease.

"He will be prosecuted" when he returns to Liberia, Binyah Kesselly, chairman of the board of directors of the Liberia Airport Authority, told reporters.

He said people like Mr Duncan and Patrick Sawyer, a Liberian-American with Ebola who travelled to Nigeria and infected people there, have brought a "stigma" upon Liberians living abroad.

More than 7,100 people have contracted the virus in West Africa and more than 3,300 have died after contracting it, according to the World Health Organisation (WHO).

A woman who Mr Duncan visited has been confined to her apartment under armed guard under a confinement order after the family failed to comply with a request to stay home, according to Dallas County Judge Clay Jenkins.

American Red Cross representatives delivered food to the apartment Thursday and the North Texas Food Bank said it sent cereal, tuna, produce and other supplies.

The confinement order, which also bans visitors, was imposed after the family failed to comply with a request to stay home, according to Dallas County Judge Clay Jenkins. Texas State Health Commissioner David Lakey said the order would ensure the woman, her 13-year-old son and two nephews can be closely monitored for signs of the disease.