The director of the U.S. Centers for Disease Control and Prevention said on Thursday he has activated the agency's emergency operation center at the highest response level to help respond to the worst Ebola outbreak in history.

In testimony at a special congressional hearing on Ebola, CDC Director Dr Thomas Frieden said the CDC has more than 200 staff members in Atlanta working on the outbreak, and will soon have more than 50 disease experts in West Africa to try to contain the outbreak.

Although Frieden said it is possible that people who have traveled to West Africa might bring the virus back home with them, and even spread it to some healthcare workers and family members, he said, he is "confident there will not be a large Ebola outbreak in the United States."

Frieden said it is not clear whether experimental treatments that were given to the two infected U.S. aid workers now in the United States who are being cared for at Emory University in Georgia will ultimately be effective, and he emphasized that the best approach to this threat is to contain it.

"In terms of the promising drugs, I can assure you that the U.S. government is looking into this very carefully," Frieden said. "But I don't want there to be false hope out there. Right now, we don't know if they work," he said.

Frieden said from what he has heard, there are only a handful of doses of the drugs given to the U.S. patients at the moment. And he said that the experiences of the two patients who got those drugs would not be sufficient to determine whether the treatment worked. (Reporting by Julie Steenhuysen; Editing by James Dalgleish)