Ebola Transmission: An American cameraman in Liberia was diagnosed with Ebola

A cameraman working in Liberia has tested positive for Ebola and is to be flown home to the US for treatment.

The 33-year-old freelancer has been working in the country for three years for a number of media outlets, most recently NBC News.

More than 3,330 people have died in four West African countries in what has become the world's worst outbreak.

President Barack Obama has pledged federal support to contain the spread in the US, after the first case there.

A Liberian man diagnosed in Texas on Tuesday remains in a serious condition.

The unnamed cameraman is the fourth American known to test positive for Ebola, all diagnosed in Liberia.

Three American aid workers were separately flown back to the US for treatment and they are all recovering.

The man was only hired by NBC News on Tuesday, the US broadcaster said, and he came down with symptoms - including fever and aches - the following day.

After seeking medical advice, he tested positive for the virus.

NBC News President Deborah Turness informed staff of the news in a statement.

"We are doing everything we can to get him the best care possible. He will be flown back to the United States for treatment at a medical center that is equipped to handle Ebola patients."

The rest of the NBC crew including the network's chief medical editor, Dr Nancy Snyderman, are being flown back to the US on a private charter flight and will be placed under quarantine for 21 days, Ms Turness added.