The first Ebola patient to be diagnosed in the United States, Thomas Duncan, has seen his condition worsen from "serious" to "critical," the Texas Health Presbyterian Hospital said.

"Mr. Duncan is in critical condition," a brief news release from the hospital said.

Duncan, who flew from Liberia to Dallas last month is currently being kept in isolation. Health officials say they are monitoring nearly 50 people who may have had contact with Duncan for signs of Ebola, thus far none have exhibited symptoms.

"We are confident that none of those with definite contact had any symptoms related to Ebola, none of them had fever," director of the US Centers for Disease Control and Prevention (CDC) Tom Frieden said.

US health officials have had nearly 100 inquiries regarding potential new cases of Ebola since Duncan was diagnosed with the deadly disease. Thus far, now new infections have been found, health officials said.

Meanwhile, officials from the Centers for Disease Control and Prevention (CDC) rushed to Newark Liberty International Airport in New Jersey to escort two passengers off a plane that was arriving from Brussels after they displayed possible symptoms of Ebola, local media reported.

The two passengers, a man and his daughter who are believed to be from Liberia, were said not to be sick with Ebola, local media said citing CDC officials. All the other passengers were cleared to leave the plane.

Ebola has killed more than 3,400 people since the outbreak began in March, according to the World Health Organization.

bw/rc (Reuters, AP, AFP)