(Recasts, adds details, French extradition interest)

By Alberto Dabo

BISSAU, Jan 11 (Reuters) - Police in Guinea-Bissau on Friday arrested two suspected members of al Qaeda wanted for the killing of four French tourists who were shot in Mauritania on Christmas Eve, the local police chief said.

"I can confirm that we have two Mauritanians suspected of killing the French tourists in Mauritania. They were arrested early this morning at a hotel in Bissau," Judicial Police Director Lucinda Ahukharie told Reuters.

She said police in the former Portuguese colony had carried out the arrests at the request of French authorities, who had helped them. French anti-terrorism police had already opened their own investigation into the killings.

On Dec. 24 three attackers, who Mauritanian officials said were suspected of being Islamic militants linked to al Qaeda, shot dead four French tourists and injured a fifth as they enjoyed a Christmas Eve picnic on a road in southern Mauritania.

The killings shocked the largely desert former French colony, which straddles Arab and black Africa. Mauritania has been trying to develop desert tourism and tourists from France were the most frequent visitors.

The killings raised fears that Islamic extremists linked to al Qaeda, who have carried out attacks in Morocco and Algeria, might be trying to extend their operations southwards into sub-Saharan Africa. The shootings led to the cancellation of the 2008 Lisbon-Dakar rally due to take place in January.

Guinea-Bissau, where the two Mauritanians were arrested, lies to the south of Mauritania's southern neighbour Senegal.

Ahukharie named the men detained in Bissau's Lobato Hotel as Mohamed Ould Chabarnou and Sidi Ould Sidna and described them as "presumed al Qaeda terrorists".

Guinea-Bissau is awaiting formal extradition requests from Mauritania or France. "French authorities have already expressed their interest in seeing the alleged killers of their citizens extradited to France," she said.

AL QAEDA LINKS

Ahukharie said one of the Mauritanians under arrest spoke the local creole language well and had previously spent two years in Guinea-Bissau. The small West African state has recently become a transit point used by international drug cartels to take Colombian cocaine to Europe in boats and planes.

It was not clear whether the arrested men had any links with the drug trade.

Mauritanian security forces have already detained at least seven people suspected of helping the killers.

Three days after the French tourists were slain, gunmen killed three soldiers in remote northern Mauritania, bordering Algeria and Morocco's breakaway territory Western Sahara.

In an audio recording aired by Al Arabiya television soon afterwards, a spokesman for al Qaeda's North African branch said the group had killed the Mauritanian soldiers, but made no mention of the French.

Mauritanian prosecutors had said at least two of the three suspected killers of the French tourists were linked to an Algeria-based Islamic militant group formerly known as the Salafist Group for Preaching and Combat (GSPC).

The GSPC has changed its name to al Qaeda in the Islamic Maghreb after allying itself to mainstream al Qaeda. In September, al Qaeda's second in command urged north African Muslims to "cleanse" their land of Spaniards and French. (Additional reporting by Pascal Fletcher and Diadie Ba in Dakar, writing by Pascal Fletcher, editing by Tim Pearce)

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