Malian soldiers fired on Islamist fighters on Tuesday, according to reports.
The BBC cited military sources saying that Islamist fighters, who control a vast portion of the north of the country, were advancing towards Mopti, the last town still controlled by the government in the region.
Military sources said the army responded to the advance with "warning shots."
GlobalPost's senior correspondent Tristan McConnell reported from Nairobi: "Mali's Islamists appear to be flexing their muscles as momentum towards a United Nations-backed regional military intervention slowly gathers. In recent days they have moved southwards towards the cinched-waist of the country that marks the division between the government-controlled south and Islamist-controlled north."
"One Islamist faction, Ansar Dine, has also announced the end of a ceasefire it had agreed with the government as part of regional talks," noted McConnell.
The Islamists gained control of large swathes of the northern half of Mali in April, following a March coup that overthrew the democratically elected president and left behind a power vacuum. The Associated Press reported that Ansar Dine has been implementing Islamic law in the north, carrying out punishments such as public executions, amputations and whippings.
In December, the UN Security Council approved a plan to send 3,000 West African troops to help retake the north, the BBC noted.
McConnell said, "Mali's troubles are looking as intractable as ever, a solution still far off."
Follow Tristan McConnell @t_mcconnell, who contributed reporting from Nairobi.