MONROVIA: The UN launched a mission on Thursday to prevent the worldwide spread of Ebola as the US hunted for people who came in contact with the first African diagnosed with the deadly virus outside the continent.

Anthony Banbury, the special representative for the UN Mission on Ebola Emergency Response (UNMEER), was expected to set ambitious targets for action on the crisis as he began a tour of the three worst-hit nations in the Liberian capital Monrovia.

President Ellen Johnson Sirleaf said she had told Banbury the virus had spread to all 15 counties of Liberia, the worst-hit nation with almost two-thirds of the 3,338 deaths in west Africa.

"Affected people are leaving from urban places and hiding out in remote communities," Sirleaf said, according to a statement from the presidency following the meeting on Wednesday.

"If we do not move in as quickly as possible, the virus will further spread in rural areas."

Banbury was due to address the media before moving on to Sierra Leone and then Guinea over the coming days, with US health officials scouring the Dallas area for people who came in contact with a Liberian man diagnosed with Ebola.

The man first sought treatment in Texas on Sept. 25 but hospital officials have admitted he may have come into contact with many more people than first thought because an apparent miscommunication among staff resulted in his release back into the community for several days.

Ebola is spread through close contact with the bodily fluids of an infected person, and can only be transmitted when a patient is showing symptoms like fever, aches, bleeding, vomiting or diarrhoea.

The man -- the first person to be diagnosed with Ebola on US soil -- flew from Liberia, the hardest-hit nation in west Africa's deadly Ebola outbreak, and arrived in Texas Sept. 20 to visit family. He fell ill on Sept. 24.

He went to the hospital the next day but was sent home because the medical team "felt clinically it was a low-grade common viral disease", said Mark Lester, executive vice president of Texas Health Resources.

"He volunteered that he had travelled from Africa in response to the nurse operating the checklist and asking that question," Lester added.

"Regretfully, that information was not fully communicated throughout the full team."

The patient is currently in serious but stable condition.

The Liberian government expressed "regret" on Thursday over the spread of Ebola from Monrovia to the US, adding that the incident had demonstrated "the clear international dimension of this Ebola crisis".