At least six Americans have been exposed to the Ebola virus during a deadly outbreak in the Democratic Republic of Congo, sources have told CBS News, the BBC's US partner.

The World Health Organization (WHO) has declared the outbreak an international emergency, with the DR Congo's health ministry reporting around 350 suspected cases and 91 deaths.

One American is believed to have symptoms, with three others said to have faced a high-risk contact or exposure. It is unclear whether any have been infected.

The US Centers for Disease Control and Prevention (CDC) said it was supporting the "safe withdrawal of a small number of Americans who are directly affected", but did not confirm how many.

The current strain of Ebola is caused by the Bundibugyo virus, for which there are no approved drugs or vaccines.

There are also two confirmed cases and one death in Uganda, according to the CDC.

The US government is reportedly looking to arrange transport for the small group of Americans in DR Congo to a safe quarantine location, a source told health news site STAT.

Quoting a source, the site adds that the group could be taken to a US military base in Germany, though this has not been confirmed.

During a press conference on Sunday, the CDC declined to answer direct questions about the US citizens reportedly affected, adding that the risk to the US remained low.

The US has also issued a Level Four travel advisory - its most severe level - warning against travel to the DR Congo.

The WHO has said the outbreak in DR Congo's eastern Ituri province is a public health emergency of international concern, but did not meet the criteria of a pandemic.

The agency has also warned it could potentially be "a much larger outbreak" than what is currently being detected and reported, with significant risk of local and regional spread.

More than 28,600 people were infected by Ebola during the 2014-2016 outbreak in West Africa, the largest outbreak of the virus since its discovery in 1976.

The disease spread to a number of countries within and outside of West Africa, including Guinea, Sierra Leone, the United States, the United Kingdom and Italy, killing 11,325 people.

Jean Kaseya, director general of Africa Centres for Disease Control and Prevention, warned that in the absence of vaccines and effective medicines people should follow public health measures, including the guidance about handling the funerals of those who have died from the disease.

"We don't want people infected because of funerals," he told the BBC World Service's Newsday programme.

Community funerals, where people helped wash the bodies of their loved ones, contributed to many people becoming infected in the earlier stages of the big outbreak more than a decade ago.

The WHO has advised DR Congo and Uganda, two countries with confirmed cases, to undertake cross-border screenings to avoid the virus spreading.

It has also urged nearby countries to "enhance their preparedness and readiness", including surveillance at health facilities and communities.

Neighbouring Rwanda said it would be tightening screening along its border with DR Congo as a "precautionary measure", while Nigeria said it was "closely monitoring the situation".