President Nicolas Sarkozy vowed Monday that France would avenge the murder of a 78-year-old aid worker who was kidnapped in the Sahara desert by Al-Qaeda's North African arm.

"I condemn this barbarous act, this odious act which has put an end to the life of an innocent man," the French leader said in a television address, after Al-Qaeda in the Islamic Maghreb (AQIM) announced the killing.

"Dear compatriots, this crime committed against Michel Germaneau will not go unpunished," he added, warning French nationals to avoid all travel to the arid region running through Mauritania, Mali, Niger and southern Algeria.

"We demand instantly of our countrymen that they abandon absolutely all travel in the Sahel zone," he said, adding that AQIM had previously killed a British hostage and has "no respect for human life."

Sarkozy said he had instructed his defence and foreign ministers to take steps to prepare France's response, and confirmed that French soldiers took part in a raid last week against an Al-Qaeda base in Mali.