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 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 health agency 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 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.