"The first effect of this agreement is the cessation of all violence against civilians," President Faustin-Archange Touadera said at the signing ceremony, without disclosing the contents of the pact.

"For us, this day is a historic moment which enshrines the culmination of nearly three years of efforts," Touadera said, adding he was "holding out a hand to our brothers and sisters in the armed groups."

The deal had been initialled in Khartoum on Tuesday after lengthy talks brokered by the African Union and supported by the United Nations.

The leaders of the two biggest armed groups -- Ali Darassa of the Unity for Peace in Central African Republic (UPC) and Noureddine Adam of the Popular Front for the Renaissance of the Central African Republic (FPRC) -- initialled the text in Khartoum but were not present for Wednesday's the signing ceremony.

The accord is the eighth attempt in nearly six years to forge peace in the war-ravaged country -- one of the poorest in the world.

The conflict has left thousands dead and forced a quarter of the population of 4.5 million from their homes. The rural exodus, the UN warned last year, could drive the country into famine.

All seven previous peace agreements failed to stick.

The last deal, forged in 2017 with the help of the Catholic Church, lasted less than a day before violence erupted anew, claiming nearly a hundred lives.

"The real challenge will be following up this agreement," the head of the AU Commission, Moussa Faki Mahamat, cautioned on Wednesday.

"It should not be the umpteenth (failed) agreement, as cynics say," he added. "We will be very watchful about the effective application of it."

The CAR plunged into crisis in 2012, when a mainly Muslim rebel movement called the Seleka rose in the north of the country.

The following year, the insurgents overthrew President Francois Bozize, a Christian, triggering a predominantly Christian militia called the anti-Balaka.

France, the former colonial ruler, intervened militarily under a UN mandate as fears grew of a Rwandan-style genocide.

The Seleka were forced from power and in February 2016, Touadera, a former prime minister, was elected president.

However, militia, often portraying themselves as defenders of a community or religious group, still control some 80 percent of the country.

Fighting often erupts between these forces over control of the country's lavish mineral resources, which range from gold and diamonds to uranium.

Touadera is supported by 13,000 troops and police in the United Nations' MINUSCA mission, one of the UN's biggest peacekeeping operations.