The clash is the latest sign of an escalation in the battle between Saharan countries and Al-Qaeda in the Islamic Maghreb (AQIM), the chief suspect in the kidnapping of seven foreigners, including five French citizens, in Niger on Thursday.
The fighting continued on Saturday as Mauritanian forces encircled around 20 AQIM vehicles in the border area, the Mauritanian source told Reuters. In addition to those killed, four Mauritanian troops were also wounded in the fighting.
"The operation was launched because the opportunity presented itself. It was not planned long in advance," said a second security source in the Mauritanian capital Nouakchott who knew the background to the mission.
The French Foreign Ministry said there was no connection to Thursday's kidnappings, in which a French employee of nuclear firm Areva and his wife were seized.
"This action is independent of the kidnapping of Areva employees. There are no French forces on the ground," a ministry spokesman said in Paris.
The seven hostages are believed to have been taken to Mali, a source in the Nigerien military told Reuters after an army search operation on Friday.
Security experts say Al-Qaeda allies are building a base in the desert region that straddles the porous and thinly policed borders of Algeria, Mali, Niger and Mauritania after being squeezed out of traditional patches along Algeria's coast.
French President Nicolas Sarkozy vowed in July to to punish AQIM for executing a 78-year-old French hostage after a joint Franco-Mauritanian commando raid failed to free him. AQIM in turn has said it would avenge its fighters killed in the raid.
The group has not claimed responsibility for Thursday's brazen kidnapping of foreigners in Niger's northern uranium mining zone, which has raised fresh doubts about the security of Western mining operations in the region.
Neither France nor the mining firms gave details of how the workers were seized but a local businessman and a source in the mining industry in Niger told Reuters the foreigners were taken while they slept in their houses in the town of Arlit.
Among the hostages were five employees of Vinci , whose subsidiary Sogea-Satom is a contractor in the region.
The French Foreign Ministry urged its nationals on Friday to quit danger zones in Niger and an Areva executive said the company had called some of its staff back from Arlit. Areva employs 2,500, the bulk of them locals, at three mine sites in Niger. Niger hopes to become the world's No. 2 uranium producer when the Imouraren mine comes on line in 2013 or 2014.
