A massive explosion near Gao in northern Mali late Saturday raised fears of rebel's attack, hours after villagers near the city detained two youths preparing suicide bombings.

The blast late Saturday was audible from the northern Malian city of Gao, which French-led forces only recaptured from the rebels late last month.

It went off at around 11:00 pm (2300 GMT), an AFP correspondent said.

A French military source said it had happened some 10 kilometres (six miles) away from the French military base at the city's airport, but could provide no further details.

At about half past midnight Sunday, French military helicopters could be heard around Gao.

French-led forces took back Gao, the major city in northern Mali, on January 26 after a campaign of French air strikes forced Islamist fighters there to flee.

But while some have fled into the desert wilderness further northeast, others have stayed in the bush surrounding the city, mingling with local residents sympathetic to their cause, French and Malian military sources say.

One Malian officer recently told AFP that it is still dangerous for his troops to venture too far outside the city.

At dawn on Friday, a young suicide bomber blew himself up, slightly injuring a Malian soldier at the entrance to the city. It was the first attack of its kind in Mali.

And on Saturday morning, two young men carrying explosive belts were arrested near the city before they could set off their devices.

Troops in the area have also been discovering landmines left by the insurgents. In recent days mines have killed two Malian soldiers in one incident and four civilians in another.

Gao lies 1,200 kilometres (750 miles) northeast of the capital Bamako and is a key point of access for the desert wilderness that lies further northeast, towards Mali's border with Algeria.