The attack took place on Friday between the northern cities of Gao and Ansongo, they said.
The death toll was initially put at 10 but soon rose to 32.
"We have more than 30 bodies from the scene," said a hospital source in Gao.
"The jihadists ambushed a civilian convoy escorted by Malian soldiers and Wagner mercenaries," a local official told AFP, requesting anonymity.
"There are civilians and soldiers among the dead."
A medical source said many of the dead and wounded had been transferred to Gao, the main city in northern Mali.
A source from a transport trade union said: "According to a transporter who managed to escape, jihadists ambushed the convoy's escort and opened fire on everyone at random to cause the largest number of victims."
Another local official told AFP: "The Malian soldiers and Wagner (mercenaries) were in around 10 vehicles protecting a convoy of 22 minibuses with civilian passengers, six large buses and eight lorries."
"At least five lorries were destroyed by Islamic State (IS) jihadists."
IS has not claimed responsibility for the attack.
The Malian army has not officially commented on the reported attack.
"We control the situation on the ground between Ansongo and Gao," a military source said.
The route between Ansongo and Gao has seen several attacks in recent months blamed on jihadists or bandits.
- Gold mine -
Another local official said the civilian victims were mainly foreigners travelling to a gold mine in Intahaka, the main gold mining region in northern Mali.
The country is one of Africa's top gold producers, though production has plunged recently.
Mali has faced serious security problems since 2012 linked to violence both by groups linked to Al-Qaeda and IS and by local criminal gangs.
In January the Malian army said it had arrested a top IS leader and killed several "jihadist" fighters in an operation in the Gao region.
It named the suspect as Abou Hach, a "wanted terrorist long known to the intelligence services".
The impoverished west African country has been plunged into instability by a series of coups since 2012 and has struggled to deal with the security crisis in the north.
Its military rulers have broken ties with former colonial ruler France and turned politically and militarily towards Russia.