The US State Department ordered all non-essential diplomats and staff to leave its embassy in Chad because of the possibility of insurgent attacks on the capital.
The families of American personnel stationed there were also instructed to leave because armed groups appeared to be advancing.
“Armed non-governmental groups in northern Chad have moved south and appear to be heading towards [Chadian capital] N’Djamena,” the State Department said in a travel alert.
“Due to their growing proximity to N’Djamena, and the possibility for violence in the city, non-essential US government employees have been ordered to leave Chad by commercial airline.”
The State Department has often warned US citizens not to travel to Chad because of unrest and the presence of ISIS-aligned terrorist insurgency Boko Haram.
Officials said any Americans there now who wanted to leave Chad should do so.
Britain also urged its citizens to leave the country as soon as possible.
Chad's army said on Saturday that it had "completely destroyed" a column of rebels that attacked the country on the day of last week's presidential election.
Soldiers were searching for the last of the rebels, army spokesman Azem Bermandoa Agouna said in a statement read on national television.
"The adventure of the mercenaries from Libya has ended, as announced," Communications Minister and government spokesman Cherif Mahamat Zene announced on Twitter.
"Congratulations to our valiant defence and security forces."
L'aventure des mercenaires en provenance de la Libye a pris fin comme annoncée. Félicitations à nos vaillantes forces de défense et de sécurité. Le porte-parole de l'armée communiquera incessamment le bilan de combats.
— Chérif Mahamat Zene (@Cherif_MZ) April 17, 2021
Four tanks and several soldiers were stationed at the northern entrance to N'Djamena on Saturday evening, where military vehicles continued to drive towards the fighting, an AFP journalist said.
Last Sunday, members of the Force for Change and Concord in Chad (FACT) rebel group said they captured garrisons near Chad's northern borders with Niger and Libya "without resistance".
FACT is based in Libya, where it has a non-aggression pact with Field Marshall Khalifa Haftar, whose Libyan National Army controls much of the country's east.
Mainly made up of the Saharan Goran people, FACT clashes regularly with the Chadian army.
The latest assault came last Sunday, the day of Chad's presidential election, which the incumbent Idriss Deby – who has ruled the country for 30 years – is expected to win.
Preliminary results will not be announced until April 25.
Initial indications suggested large numbers of people stayed away from the vote, prompting opposition figures to claim their calls for a boycott of the polls had worked.
The Tibesti Mountains near the Libyan frontier are frequently the place where rebels and the Chadian army fight. French air strikes were called to stop an incursion there in early 2019.
Landlocked Chad is home to nearly half a million refugees from neighbouring Sudan, Nigeria and Central African Republic.
Another 330,000 Chadians are internally displaced, the majority in the volatile Lake Chad region, where Boko Haram militants are active.
