Malian soldiers have fired on Islamist fighters in the centre of the country, military sources say.
Details remain sketchy but some sources say the Islamists, who last year seized a vast area of the north, have advanced towards Mopti - the last town under government control.
The army responded with "warning shots", military sources say.
In December, the UN approved an African-led military operation against the Islamist groups.
Last week, one of the Islamist groups, Ansar Dine, said it was ending the ceasefire it had previously announced during talks with mediators.
The army used artillery against the Islamist fighters in the village of Gnimignama, 30km (19 miles) from army positions, according to army sources.
"Jihadist elements" are now deployed on several points along the frontier between the two sides, from the Mauritanian border in the west to the Douentza region in the east, Malian Defence Minister Col Yamoussa Camara told Radio France International.
It is not clear whether there were any casualties.
Representatives of the Malian government and Islamist and Tuareg rebels are due to hold talks in neighbouring Burkina Faso on 10 January.
The rebels seized power in the north - an area the size of France - in the chaos following an army coup in March.
Since then, there has not been any fighting between the army and Islamist forces.
The alliance between the Islamists and Tuareg quickly collapsed, with the Islamists taking the region's main urban centres.
The Islamist groups have since destroyed ancient shrines in Timbuktu and imposed a strict interpretation of Islamic law, sparking international outrage.
The UN Security Council last month gave its backing for a plan to send some 3,000 West African troops to help Mali's government retake the north if no peaceful solution could be found in the coming months.
No operation is expected to begin before September 2013.