Four members of an Al-Qaeda linked group went on trial here Tuesday for kidnapping three Spanish aid workers in Mauritania last year.

The trial got underway in the capital Nouakchott while two of the Spaniards are still being held by Al Qaeda in the Islamic Maghreb (AQMI) in the northern Malian desert.

Among the defendants in court was a 52-year-old Malian national, Omar Sid-Ahmed Ould Hamma, believed by investigators to be the key figure behind the kidnappings carried out in November last year.

However, most of the 11 accused have evaded arrest and are being tried in absentia.

The three Spaniards, two men and a woman working for a Barcelona-based NGO, were travelling in the last vehicle of a humanitarian convoy when they were abducted between Nouakchott and Nouadhibou on November 29.

Authorities claim Hamma, nicknamed Omar the Sahrawi, carried out the kidnapping himself along with two other Malians who are being tried in absentia.

Prosecutors say the kidnap gang were paid mercenaries of one of AQMI's leaders, an Algerian named Mokhtar Belmokhtar.

Investigators say the Spaniards were taken through Moroccan-controlled territory in the Western Sahara and into a desert region in the extreme north of Mali.

Female hostage Alicia Gamez was released on March 10 after three months in captivity, but her two countrymen and an Italian couple -- kidnapped in December -- are still in the hands of the Islamist group.

The kidnappers have demanded payment of several million dollars in ransom and the release of Islamist prisoners held in Mauritania, which along with neighbouring desert states Mali and Niger has witnessed a rise in terrorism-related kidnappings of westerners.

Hamma was arrested in February in an operation by Mauritanian security forces on the border with Mali.

Terrorism experts believe AQMI has raked in millions of dollars from ransoms to fund a tiny but well-drilled army whose influence now spans large parts of west Africa.

The kidnapping of tourists, which began in 2003 when 32 German and Swiss travellers were seized in southern Algeria, has become big business for local armed gangs who sell on their "catch" to AQMI.