The U.S. government has advised its citizens to avoid all non-essential travel to three countries in west Africa that have been hit by an outbreak of Ebola.

The Atlanta-based Center for Disease Control (CDC) announced the advisory covering Guinea, Liberia, and Sierra Leone, not only for the protection of U.S. citizens, but also to limit their use of overburdened hospitals and clinics in the area.

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The alert becomes the highest-level warning since the Ebola outbreak spread among the three African countries, where more than 700 have died of the virus this year. The last time CDC issued a high-level alert was in 2003 during the SARS outbreak in Asia.

Previous warnings from the CDC only advised travellers to exercise precaution when in west Africa. One American, Patrick Sawyer, died last week in Nigeria after having contracted the virus in Liberia. Two other Americans, helping Ebola patients in Liberia, have been infected, one of whom was reported to be in grave condition.

CDC Director Tom Frieden announced the advisory, calling Ebola a "tragic, dreadful, merciless virus."

In Washington, White House press secretary Josh Earnest said the U.S. is looking at the possibility of bringing the infected Americans back to the U.S.

No final decision has been made on this, but Earnest said private companies may be used to transport them.

Later Thursday, officials at the Emory University Hospital in Atlanta said one of the Americans in Liberia may be transferred in the next several days, according to an Associated Press report.

The hospital is near the CDC's main campus and has a special isolation unit built in collaboration with the CDC. It is one of only four facilities of its kind in the United States.

CDC says it has about two dozen staffers in west Africa helping to control the outbreak, aside from workers helping at airports to screen passengers.

CDC director Frieden said they plan to send 50 staffers more to west Africa next month.

TagsSierra Leone, Guinea