At least two children were among the dead, the French embassy in Mauritania told the AFP news agency.

The four were killed during a robbery, the police chief of the nearby town of Aleg told AP news agency.

A fifth person - said to be the father of the family - was seriously injured in the attack, about 250km (160 miles) east of the capital, Nouakchott.

The wounded man was receiving treatment in hospital in Aleg but would be transferred immediately to Nouakchott, the French embassy official added.

French President Nicolas Sarkozy said details of the incident were not yet clear.

"It's still too early to say exactly what the reasons were for this attack which I can't describe because we don't know the motivation or even really how it occurred," he said during a visit to a Paris hospital.

"As things stand, I can only confirm this sad news and offer my condolences."

The attack is being seen as rare in the south of Mauritania, a former French colony which enjoys relative stability. The north of the country is generally regarded as less safe for travellers.

Demands

The five victims had stopped on the side of a road for a picnic when the gunmen approached demanding money, Aleg police chief Mohamed Ould Lemine told AP.

The gunmen took their money and then opened fire with automatic weapons, before speeding off in a car, the police chief said.

The car was later reported to have been found in Aleg, and police were said to be conducting house-to-house inquiries in the town.