France has declared war on al-Qaeda, after launching its first attack on one of the group's bases in North Africa.

"We are at war with al-Qaeda," Francois Fillon, the French prime minister, said on Tuesday, several days after special forces carried out a strike against an al-Qaeda base on the Mauritania-Mali border.

The attack was the country's first strike against an al-Qaeda base, and six fighters were reportedly killed.

French officials added that the attack last week was a last-ditch effort to save Michel Germaneau, a 78-year-old aid worker al-Qaeda said it had killed. 

The declaration and attack marked a shift in strategy for France, which is usually discreet about its military co-operation with its regional allies - Mauritania, Mali, Niger and Algeria.

The al-Qaeda franchise was spawned amid an Islamist insurgency in North Africa and the Salafist Group for Call and Combat formally merged with al-Qaeda in 2006, naming themselves al-Qaeda in the Islamic Maghreb (AQIM), with its fighters spread through the Sahel region, a desert expanse as large as the European Union.

Executed over raid

Al-Qaeda said it killed the aid worker in retaliation for last week's raid [AFP]

In April, Algeria, Mauritania, Mali and Niger opened a joint military headquarters deep in the desert to respond to threats from traffickers and the al-Qaeda offshoot.

US special forces have also helped the four nations train troops in recent years.

However, for the French, the military operation may have backfired.

The al-Qaeda group said in an audio message broadcast on Sunday that it had killed Germaneau in retaliation for the raid.

An estimated 500 fighters are thought to roam the Sahel region. Despite the meagre numbers, the region's fighters pose a clear threat.

Among the more recent victims, a British captive was beheaded last year and two Spanish aid workers were taken hostage in Mauritania in November.

Mauritanian soldiers also have been killed in numerous attacks.