By Ibrahima Sylla

NOUAKCHOTT (Reuters) - Gunmen travelling in two vehicles killed three Mauritanian soldiers on Thursday in a desert clash in the north of the Saharan country, a military official said.

The identity of the gunmen was not immediately known but the clash took place in the same vast northeastern desert region of Mauritania where Islamic militants allied to al Qaeda killed 15 soldiers in a 2005 raid. The region borders Western Sahara to the west, Algeria to the north and Mali to the east.

On Monday suspected Islamic extremists killed four French tourists and wounded a fifth in southern Mauritania in an attack that raised fears al Qaeda-linked militants who have carried out attacks in Algeria and Morocco may be seeking to extend their operations southwards to Mauritania, Senegal and Mali.

Another security official had earlier reported that four soldiers were killed in Thursday's incident near an army position at El Ghallawiya, some 700 km (440 miles) northeast of the capital Nouakchott.

The Mauritanian military official, who asked not to be named, said a unit from the El Ghallawiya army outpost had tried to intercept two vehicles travelling through the desert area, not far from the huge Guelb er Richat crater.

"The fleeing occupants of the cars turned and opened fire on the army vehicle, killing three soldiers and seizing a weapon," the official said. The army was hunting for the attackers.

The killing of the French tourists and the slaying of the soldiers occurred as Mauritanian authorities were preparing to host the high-profile Lisbon-Dakar rally that will start January 5 and cross through Morocco and Mauritania en route to Senegal.

Traffickers of drugs and arms, Tuareg rebels and armed bandits also roam the huge desert wastes of the southern Sahara that extend across the north of Mauritania, Mali and Niger.

Security forces in Mauritania, Senegal and Mali are hunting for the three killers of the French tourists, two of whom are suspected of links with the Algeria-based Islamic militant group formerly known as the Salafist Group for Preaching and Combat (GSPC), which is accused of attacks in North Africa.

The GSPC has changed its name to al Qaeda in the Islamic Maghreb after allying itself to mainstream al Qaeda. In September, al Qaeda's second in command urged north African Muslims to "cleanse" their land of Spaniards and French.

Monday's killings shocked the largely desert former French colony that straddles Arab and black Africa. Authorities have been trying to develop a nascent desert tourism industry. (Additional reporting by Diadie Ba; Writing by Pascal Fletcher; Editing by Ibon Villelabeitia)