French Pierre Camatte has been held since November
Mauritania has temporarily recalled its ambassador to Mali, angered over the release of four militants from al-Qaeda's North African branch.
Nouakchott said the release of the fighters - one of whom is Mauritanian - violated security pacts.
Mali freed the men weeks after al-Qaeda militants demanded their release, threatening to kill a French hostage.
The authorities insisted they had not given into al-Qaeda's demands, saying that the men had served their time.
Al-Qaeda in the Islamic Maghreb, as the group is known, snatched Frenchman Pierre Camatte from his hotel in Menaka, near Mali's border with Niger, on 25 November.
It is thought he is being held in the northern Mali desert, along with three Spanish hostages and an Italian couple.
The other hostages were kidnapped in neighbouring Mauritania last November.
Security sources in Mali told AFP news agency Mr Camatte could be freed within days.
But Mauritania's Foreign Ministry made clear its displeasure in a strongly worded statement.
"This non-cordial measure taken by the Malian authorities harms age-old relations between the two countries," the statement said.
"Above, all it's a flagrant contradiction of judicial co-operation accords and security co-ordination agreements."
According to local newspaper reports, two of the men were Algerian, one was Mauritanian and the fourth was from Burkina Faso.
The four were apparently convicted of illegal possession of firearms and given nine-month sentences - the exact time they had already spent on remand.
Analysts in the region claim that the Malian authorities were pressurised to release the men by the French government - allegations Bamako has repeatedly denied.