US Thursday has put on high alert the Centers for Disease Control and Prevention (CDC) Thursday against Ebola outbreak, which has killed many hundred people in West Africa.

"I have activated the CDC emergency operation centre at level one for this outbreak. This is our highest level of response," CDC director Tom Frieden told a congressional hearing

"It doesn't mean that there's an increased risk to Americans, but it does mean that we are taking an extensive effort to do everything we can to stop the outbreaks," Xinhua quoted Frieden as saying.

Calling it "a crisis" and "unprecedented," he said at the current trend there will have been more cases in this outbreak than in all previous recognised outbreaks of Ebola put together" within another few weeks.

And the best way to protect everyone worldwide is to stop the outbreak at the source in today's interconnected world, Frieden said.

"That is the only way to get control," he said. "I am confident that if we do what works, we will stop this one. But it won't be quick and it won't be easy. It requires meticulous attention to detail, because if you leave behind even a single burning ember, it's like a forest fire. It flares back up."