A jailed leader of Al-Qaeda's North African movement has threatened France and Mauritania with retaliation after forces from the two countries staged a joint military raid against one of the group's bases in Mali.

Six fighters from Al-Qaeda in the Islamic Maghreb (AQIM) were killed during a last-ditch effort to save Michel Germaneau, a 78-year-old aid worker.

"I say to the infidels and French crusaders ... will not rest until French blood has been spilled," El Khadim Ould Semane, the head of AQIM in Mauritania, said in remarks published on Monday.

In a telephone interview with the private daily Nouakchott Info, Semane said that "fighting hand in hand with French miscreants is sufficient proof that the Mauritanian army is fighting Islam".

"There are men who are prepared to retaliate [for the joint attack]."

Hostage executed

AQIM has claimed Germaneau, who was abducted on April 19, was later executed in revenge for the operation, which took place in neighbouring Mali.

Last week, Francois Fillon, the French prime minister, said France was "at war with al-Qaeda".

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.

Semane has been imprisoned in Nouakchott, the capital of Mauritania, since his arrest during an April 2008 police operation in the city, in which a policeman was killed.

AQIM, whose Mauritanian branch was officially disbanded after Semane's arrest, has also demanded the remains of those slain in the July 22 raid be handed over to their families by the Mauritanian army.

The Nouakchott Info said the interview with Semane was conducted by telephone, although telephone contact with inmates of Nouakchott prison, where 73 suspected fighters are jailed, is theoretically banned.

US warning

The United States warned its citizens on Monday to use extreme caution when travelling to Mauritania due to AQIM's increased activities.

The state department said in a travel warning that AQIM "continues to demonstrate its intent and ability to conduct attacks against foreign nationals, including US citizens".

It said that faith-based groups working in Mauritania may be particularly at riskd.

"As a result of perceived Western involvement in the raid, it is possible that AQIM will attempt additional retaliatory attacks against Western targets of opportunity," the state department said.