Central African Republic authorities have accused a former president of an attempted coup, as a coalition of rebel groups unite against the government.
It said forces loyal to François Bozizé were near the city of Bossembélé and planned to march on the capital Bangui.
Tensions have risen after Mr Bozizé's candidacy was rejected by the country's highest court, ahead of national elections next week.
The UN said on Friday it had deployed peacekeeping forces.
Rebel groups have seized several towns close to CAR's capital, clashing with government troops and looting property, and the UN said its troops were working to prevent a blockade of Bangui.
Mr Bozizé and his Kwa Na Kwa party came to power after a coup in 2003. But he was ousted 10 years later by the Seleka - a rebel coalition drawn largely from the Muslim minority - which accused him of breaking peace agreements.
The country has since been caught in conflict between the Seleka and the so-called "anti-Balaka" self-defence forces, who are mainly Christian.
After military intervention by France, the country's former colonial ruler, elections were held in 2016 and won by President Faustin-Archange Touadéra, who is currently seeking re-election.
But fighting among militias has continued, and the UN has blamed rebel groups for the country's instability.
Mr Bozizé returned to CAR in December 2019 after living in exile for six years in Benin, Cameroon and the Democratic Republic of Congo.
He faces UN sanctions for his alleged support of "anti-Balaka" militia groups in 2013. CAR authorities have also issued an arrest warrant against him for "crimes against humanity and incitement of genocide".
But Mr Bozizé still maintains a large following in the Central African Republic, especially in the army and the country's largest ethnic group, the Gbaya.
On Saturday, the three main rebel groups in the country announced that they had formed an alliance called the Coalition of Patriots for Change (CPC), accusing President Touadéra of trying to rig the 27 December election.
In a statement, the CPC invited "all other groups to join" and called on members to "scrupulously respect the integrity of the civilian population".
As election campaigning heats up, Facebook said earlier this week it had identified rival disinformation campaigns to influence the vote - masterminded by individuals with links to the French military and prominent Russian businessman Yevgeniy Prigozhin.
Moscow has fostered close ties in recent years with CAR, a country which has one of the world's poorest economies but is rich in resources like diamond and uranium.
Russian military advisors are currently stationed in CAR to help train government forces.
Reports by UN investigators, the US military and journalists have also documented activity in the country by the Wagner Group, a private military firm allegedly owned by Mr Prigozhin.