WASHINGTON: The United States finds reports that one of the most notorious leaders of Al Qaeda’s North African wing has been killed in fighting with French troops “very credible,” an official said on Friday.

Speaking on condition of anonymity because the report has not been formally confirmed, the US official said that if Abdelhamid Abou Zeid was indeed slain in Mali “it would be a significant blow to Al Qaeda in the Islamic Maghreb.”

“We find the reports very credible,” he said. Algeria’s independent Ennahar TV reported this week that Abou Zeid, a senior leader among the fighters of Al Qaeda in the Islamic Maghreb (AQIM), had been killed in northern Mali along with 40 other militants.

France has deployed troops to the area, backed by Chadian and Malian government forces, and has carried out airstrikes.

But French officials reacted with caution to the report, which is still being investigated by the military on the ground.

Algeria’s El Khabar newspaper reported on Friday that authorities there had carried out DNA tests to try to confirm Abou Zeid’s death.

“The security services are comparing DNA taken from two close relatives of Abou Zeid with samples taken from the remains of a body supplied by French forces,” it said.

But French government spokeswoman Najat Vallaud-Belkacem warned that reports of his death were so far unconfirmed.

“Our forces are engaged in operations which are extremely tough. I think we should be extremely cautious” with such reports, she told France 2 television.

And a source close to French President Frangois Hollande said it was up to the Mali authorities.

“We are on Malian territory. It’s up to them to identify persons targeted in military operations,” the source said.

Malian troops have arrested about 50 people on an island in the Niger river that was used as a hide-out by armed militants in northern Mali, military sources said.

On Friday morning, a correspondent on the island of Kadji, near the town of Gao, saw two boats filled with prisoners, all bearded and with shaved heads, crossing the Niger.

A Malian soldier who was escorting one of the boats said that since the beginning of the operation on Thursday about 50 people had been arrested, including foreigners from Togo and Burkina Faso. The presence of the men was condemned by local villagers, the source said.

“People told us that they also hid weapons, we’re searching. They (the prisoners) said nothing but they are going to talk,” he added.

The operation at Kadji was led by a Tuareg colonel, El Hadj Ag Gamou, according to a soldier. Colonel Salihou Maiga, commander of the paramilitary police in the Gao region, had earlier said there had been no fighting during the operation on the island.

A military spokesman says French forces are closing in on militants in rugged desert terrain in northeastern Mali.

Colonel Thierry Burkhard says a roughly 25-square kilometre zone in the Adrar des Ifoghas range near the Algerian border is the “centre of gravity” in a new operation involving 1,200 French, 800 Chadian and an unspecified number of Malian troops.

French and African troops are hunting rebels they dislodged from northern Mali’s main cities in a rapid campaign over recent weeks.

Abou Zeid, a 46-year-old whose real name is Mohamed Ghedir, was seen in the cities of Timbuktu and Gao after the militants took control last year and his presence stoked fears the region could become a haven for extremists.