WASHINGTON (AFP) - Texas health authorities Wednesday were investigating a potential second US case of Ebola, in a person who was in close contact with a man diagnosed with the virus.

The first diagnosed case in the United States was announced on Tuesday. The man had travelled from Liberia to visit family in the Dallas area, the US Centres for Disease Control and Prevention said.

"Let me be real frank to the Dallas County residents, the fact that we have one confirmed case, there may be another case that is a close associate with this particular patient," said Zachary Thompson, the director of Dallas County Health and Human Services, in an interview with local media WFAA. "So this is real. There should be a concern, but it's contained to the specific family members and close friends at this moment."

Three ambulance workers who transported the man to the hospital have been tested for Ebola but their blood showed no evidence of the virus, according to the city's Twitter account.

The CDC has said it is monitoring a "handful" of people who were in close contact with the man during the several days when he showed symptoms of Ebola but had not yet been hospitalized. The patient is currently in serious condition, said Texas Health Presbyterian Hospital Dallas.

The fast-growing Ebola outbreak in West Africa has killed more than 3,000 people since the start of the year.