Washington DC - The US said Wednesday it was instituting a "Do Not Board" order for American citizens who have traveled to the Democratic Republic of Congo, an effort to prevent Ebola's spread.

The order will prevent passengers from boarding commercial flights to the US unless the individual has spent 21 days outside of the central African country experiencing a deadly outbreak of the virus.

"To reduce the risk of Ebola importation into the United States, CDC, under Title 42, and the Department of Homeland Security, under Title 49, are currently working together on a DO NOT BOARD process with regard to the Democratic Republic of the Congo (DRC)," the Centers for Disease and Control said in a statement.

"American citizens who are departing from the DRC may be subject to a DO NOT BOARD order. Americans are able to return to the United States 21 days after leaving the DRC."

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The statement followed US media reports that the restriction was in effect as of Monday.

The CDC did not respond immediately to an AFP request for further details.

Non-US citizens who had been in the central African country were already prevented from traveling to the US.

More than 2,000 Ebola cases, including 754 deaths, have been recorded in DRC, where the World Health Organization warns the outbreak may be two to four times larger than official figures suggest.

The outbreak has spread to five DRC provinces, according to the latest figures released by Congolese health authorities Wednesday.

There is currently no approved vaccine or treatment for the Bundibugyo strain behind this outbreak, but the first clinical trial of an antiviral drug began Tuesday, the WHO said.