The US has barred its citizens in the Democratic Republic of the Congo (DR Congo) from boarding commercial flights home until they have spent at least 21 days in another country, citing the worsening Ebola outbreak.

The restrictions, announced by the Trump administration on Monday, will place US nationals currently in DR Congo or who recently left the country on a “do-not-board” list using federal transportation powers, Reuters reported, citing a White House official.

About two dozen Americans had been preparing to fly to the US on Tuesday, according to the official, who said the State Department would assist those affected during the waiting period.

The measure comes amid a fast-spreading outbreak of the Bundibugyo strain of Ebola in DR Congo, where 1,926 confirmed cases and 702 deaths have been reported by health authorities as of July 11.

On Friday, the US Centers for Disease Control and Prevention (CDC) said an American working for a humanitarian organization in DR Congo had tested positive for Ebola. Another US citizen infected while working in the Central African nation was admitted to Frankfurt University Hospital in Germany, his employer, the evangelical Christian organization Samaritan’s Purse, said on Monday. Peter Stafford, a surgeon and leader with the Christian missionary group Serge who contracted the virus earlier in the outbreak, was also evacuated to Germany for treatment.

Uganda has reported 20 confirmed cases and two deaths. A case linked to travel from DR Congo has also been detected in France involving a doctor who returned late last month from a humanitarian mission.

World Health Organization Director-General Tedros Adhanom Ghebreyesus has said that although the Ebola outbreak in DR Congo continues to outpace the response, the risk of wider international spread remains low.

Washington had already tightened travel rules in response to the outbreak. In May, the CDC raised its travel warning for parts of the DR Congo and Uganda, while the Department of Homeland Security directed travelers arriving from the DR Congo, Uganda, and South Sudan to go through enhanced screening at designated US airports.

On Monday, the CDC announced that it had extended for another 30 days an earlier suspension on the entry of certain non-US citizens who had been in DR Congo, Uganda, or South Sudan during the previous 21 days.

Last month, a Kenyan court halted Washington’s plan to build a 50-bed facility near Laikipia Air Base to quarantine US citizens exposed to Ebola, following legal challenges and deadly protests.