Ashoka Mukpo, 33, who was working as a freelancer for NBC news, discovered he was running a fever on Wednesday, his network said, and is in quarantine in a Medecins Sans Frontieres (MSF) treatment centre.
He is the fourth American to contract Ebola in Liberia.
Anthony Banbury, head of the UN Mission on Ebola Emergency Response (UNMEER), travelled to Sierra Leone on Friday for the second leg of a tour of the three hardest-hit nations.
"The only way we will end this crisis is if we end every single last case of Ebola so there is no more risk of transmission to anyone, and when that's accomplished, UNMEER will go home," he told journalists on Thursday in the Liberian capital Freetown.
'World's highest priority'
The UN envoy said he was intent on contributing to "the highest priority for the international community for the whole world, not just the United Nations".
US health officials meanwhile were monitoring 100 people in Texas who had potential contact with a Liberian man diagnosed with Ebola. Four family members were also ordered to stay home.
The man who is the first person to be diagnosed with the deadly disease on US soil, flew from Liberia and arrived in Texas on September 20 to visit family.
By far the most deadly epidemic of Ebola on record has spread into 5 West African countries since the start of the year, infecting more than 7,000 people and killing about half of them.
The virus, spread through infected bodily fluids, can only be transmitted when a patient is experiencing the symptoms likes severe fever, vomiting, diarrhea and, in some cases, massive internal hemorrhaging and external bleeding.
The World Health Organization said in its latest situation update there was still a "significant shortfall" in capacity in west Africa, with 1,500 more beds needed in Liberia and 450 in Sierra Leone.
Around 160 health professionals pledged by Cuba to Sierra Leone arrived Thursday, reported an AFP correspondent at the airport near Freetown.
Britain has pledged 120 million to help build an estimated 700 treatment beds, fund new community treatment centers, support existing public health services and support aid agencies in Sierra Leone.
'Five infected every hour'
“Save the Children” warned that five people are being infected with Ebola every hour in Sierra Leone and demand for treatment beds is far outstripping supply.
If the current "terrifying" rate of infection continues, 10 people will be infected every hour with the deadly virus in Sierra Leone by the end of October, the London-based charity warned.
The Gambia said on Thursday it had put in place measures to cope with an outbreak of the scale of Liberia's.
"The case in Liberia is getting worse because they have not prepared for the disease but in the Gambia we are fully prepared for the disease and the awareness level amongst the population is very high," said Sanna Sambou, the country's Ebola coordinator.
Measures include screening at borders and airports and three major quarantine centers across the country, he said.
Burkina Faso said cycling race Tour Du Faso, which was to be held from October 23 to November 2, would be suspended due to fears of Ebola.
And Germany said that a Ugandan doctor who had contracted Ebola in Sierra Leone while working for an Italian non-governmental organization had been hospitalized in Frankfurt, the second Ebola patient to be treated in the country.