The seizure of the northern two-thirds of Mali by a coalition of al Qaeda-linked militants has alarmed Western and regional governments, which fear militants could use it as a base to launch attacks.
"The army has launched an offensive on Konna. Helicopters have bombarded rebel positions. The operation will continue," a senior military source in Bamako told Reuters.
Residents in the nearby garrison town of Sevare, some 60 km (36 miles) to the south, reported the arrival of military helicopters and army reinforcements, which took part in the counter-attack starting late on Thursday.
A spokesman one of the main groups forming the rebel alliance said they remained in control of Konna.
Asked whether the rebels intended to press ahead to capture Sevare and the nearby town of Mopti, the Ansar Dine spokesman, Sanda Ould Boumama, said: "We will make that clear in the coming days."
Several witnesses said foreign soldiers had arrived at Sevare's airport, helping to calm the panicked mood which gripped the town after Konna's fall. Foreign aid agencies have pulled their workers from the town.
Mali's government appealed for urgent military aid from France on Thursday after the fall of Konna, which lies some 600 km (375 miles) northeast of the capital Bamako at the crossroads between the vast and arid rebel-held north and the populous government-controlled south.
A U.N.-mandated intervention by African troops is not expected before September due to logistical and other problems.
(Writing by Daniel Flynn; editing by Richard Valdmanis)