The decision was made at the AU summit earlier this month, Smail Chergui, head of the AU's Peace and Security Commission, said at a press conference.
"On the decision of the summit to work on deploying a force of 3,000 troops to help the Sahel countries degrade terrorist groups, I think this is a decision that we'll be working on together with the G5 Sahel and ECOWAS," Chergui said.
"I think this decision has been taken because as we see, as you can recognise yourself, the threat is expanding, it’s becoming more complex."
G5 Sahel is a 5,000-member joint force already on the ground in the Sahel, and ECOWAS is the West African regional bloc.
A localised revolt that began in northern Mali in 2012 has spread to the centre of the country and to neighbouring Burkina Faso and Niger.
Around 4,000 people died in the three countries last year, a fivefold increase over 2016, according to UN figures.
The bloodshed has escalated despite the presence of a 13,000-strong UN peacekeeping force in Mali, and rattled coastal countries to the south of the Sahel.
Final decisions from the AU summit have yet to be published, but diplomats have confirmed details of the proposed Sahel deployment.
"The summit decided to deploy about 3,000 troops for a period of six months to work with the countries of the Sahel to deal with the menace that they are facing," Edward Xolisa Makaya, South Africa's ambassador to the AU, told AFP.
"It's just a sign or a show of solidarity with the people of the Sahel."
South Africa took over as AU chair at the summit and plans to host an extraordinary summit of the body on security issues in May.
Makaya said he hoped the Sahel deployment would take place "during the course of the year".
But many details of the possible deployment have yet to be worked out.
Makaya said no countries had come forward to volunteer troops, and it was also unclear how the deployment would be financed.