Update: An American doctor has tested positive for Ebola and is being taken to Germany for treatment, officials said Monday. Read more here. Our earlier story is below.

At least six Americans were exposed to Ebola virus in the Democratic Republic of Congo, sources with international aid organizations told CBS News, although it was unclear if any had been infected.

Three of the Americans faced a high-risk contact or exposure, the sources said, and one was symptomatic. It wasn't immediately clear whether the Americans are still in Congo. The health news organization STAT was first to report on the exposures.

The World Health Organization this week declared the Ebola outbreak in Congo and Uganda a "public health emergency of international concern." At least 80 suspected deaths have been reported, the WHO said Saturday.

There were more than 300 suspected cases in Congo as of Sunday, and there have been eight laboratory-confirmed cases this year, according to the Centers for Disease Control and Prevention.

The specific virus involved in this outbreak, called Bundibugyo, has no vaccine or treatment.

The CDC said it is supporting interagency partners "who are actively coordinating the safe withdrawal of a small number of Americans who are directly affected by this outbreak."

"At this time, the risk to the American public remains low," the CDC said in a statement on Sunday. "CDC continues to closely monitor the situation and has systems in place to detect and respond rapidly to potential public health threats."

The CDC has issued travel advisories for Americans traveling in Congo and Uganda, telling them to "practice enhanced precautions" and avoid people who have symptoms, which include fever, muscle pain and rash. Other symptoms are headache, vomiting, severe weakness, abdominal pain, nosebleeds and vomiting blood, according to the CDC.

Ebola virus is spread through direct contact with the bodily fluids of an infected person and does not spread through casual contact or air.

The CDC announced Monday it's adding additional screening measures and restricting people who don't have U.S. passports from entering the country if they have been in affected areas.

A U.S. State Department spokesperson said the U.S. government is working with the governments of the Democratic Republic of Congo and Uganda to contain the virus, and is also "working to rapidly mobilize support" to international partner organizations with expertise in Ebola response, including UNICEF, the International Organization for Migration and the International Medical Corps. The State Department did not directly respond to questions about how prior cuts to USAID may have affected preparedness.

The latest Ebola outbreak in eastern Congo's Ituri province is the 17th Ebola outbreak in the country since 1976. One of the worst outbreaks killed more than 11,000 people between 2014 and 2016.

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