"The deployment of Malian, Nigerien and Burkinabe troops in the G5 Sahel force began this morning," said a colonel for France's Barkhane mission, speaking on condition that only his first name of Marc-Antoine was used.
Several hundred troops have been deployed in the initial operation, codenamed Hawbi, he said.
It will "provide a show of strength and demonstrate presence" in the Mali, Burkina and Niger border regions "and impede freedom of movement, which several armed groups have enjoyed for months," Marc-Antoine said.
The world's newest joint international force, the five- nation G5 Sahel plans to number up to 5,000 military, police and civilian troops by March 2018.
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The 5,000 will comprise two battalions each from Mali and Niger and one each from Burkina Faso, Chad and Mauritania.
The force has been placed under the leadership of a Malian general, Didier Dacko, although for the time being the national contingents will not be integrated.
The idea behind the force dates back to November 2015, when countries on the rim of the Sahara grappled with an escalating wave of jihadist attacks.
French-led intervention in January 2013 largely chased out the jihadists from northern Mali, but this region, and the centre of the country, remains chronically violence-prone and attacks have spread to neighbouring countries.
Across the region -- vast, poor, arid and politically volatile -- thousands of people have died, many of them in suicide bombings, and tens of thousands have fled their homes. In areas controlled by jihadists, extreme Islamist law is enforced, with suspected thieves having hands or feet cut off.
The G5 Sahel's activities will be initially confined to Burkina Faso, Mali and Niger, where central governments have weak control over remote areas.
The first priority will be "taking back control of border areas," targeting jihadists, bandits and human traffickers are major problems, Dacko has said.
France, the G5 Sahel's most vocal backer, has 4,000 personnel in the Barkhane force.
Its role in Hawbi will be to provide "advice and support" with air and artillery, said Marc-Antoine, speaking from a forward base in the In-Tillit region in central Mali. About 100 French troops will be involved in this context.
Estimates for the first year of operations are put at 423 million euros (USD 490 million), although French officials say the budget can be brought down to around 240 million euros.
At present, it has funding pledges from 108 million euros, comprising 50 million from the European Union, 50 million from the five countries themselves and eight million from France, plus USD 60 million promised last week by the United States.
A donor conference will be held in Brussels on December 14.
They include setting up a United Nations support office in the Sahel and sharing resources from MINUSMA, the 13,000- strong peacekeeping mission in Mali.
He also recommended extra financial backing for equipment including heavily reinforced vehicles and observation capacities, including drones.
But on Monday the US ambassador to the United Nations, Nikki Haley, said UN resources, already strained, should not be further overstretched, nor should be they used for non-UN missions.
"We believe that the G5 force must be first and foremost owned by the countries of the region themselves," she told the Security Council.