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Gunfire resounded through the sandy streets and mud-brick houses of the ancient town on the Niger River, hours after French and Malian forces reinforced a checkpoint that had been attacked for the second time in two days by a suicide bomber.
French helicopter gunships clattered overhead.
"Islamists who have infiltrated the town are trying to attack our positions, but we're fighting back," a Malian army officer told Reuters by phone. Another Malian soldier said one group of rebel infiltrators had come in on motorbikes.
Civilians crouched for cover behind walls lining narrow dusty alleys as French and Malian troops, backed by armoured vehicles, fired on the al Qaeda-allied insurgents who had slipped into the area of the central market and police station.
A Reuters reporter saw one body crumpled over a motorcycle.
A fast-moving military intervention France launched last month in its former Sahel colony has driven al Qaeda-allied fighters from Mali's main northern towns, such as Gao and Timbuktu, into the northeast Adrar des Ifoghas mountains.
But with Mali's weak army unable to secure recaptured zones, and the deployment of a larger African security force slowed by delays and kit shortages, there are fears the Islamist jihadists will hit back with more guerrilla raids and suicide bombings.
Abdoul Abdoulaye Sidibe, a Mali parliament deputy from Gao, said the rebel infiltrators were from the MUJWA group which had held the town until French forces liberated it late last month.
"There was a whole group of them who took up positions in front of the police station and started firing in all directions. But they're cornered by the troops now," he said, speaking from Bamako and citing reports from witnesses in Gao.
MUJWA is a splinter faction of al Qaeda's North African wing AQIM which, in loose alliance with home-grown Malian Islamist group Ansar Dine, held Mali's main northern towns of Timbuktu and Gao for 10 months until the French offensive drove them out.
The Islamists posted black banners with inscriptions from the Koran in the occupied towns. In the Gao gunbattles on Sunday, a Reuters TV cameraman saw a figure in black robes and a black turban, apparently one of the rebels, running to avoid heavy the fire from the Malian soldiers.
Late on Saturday, an army checkpoint in Gao's northern outskirts came under attack by a group of Islamist rebels who fired from a road and bridge that lead north through the desert scrub by the Niger River to Bourem, 80 km (50 miles) away.
BEARDED SUICIDE BOMBER
"Our soldiers came under heavy gunfire from jihadists from the bridge ... At the same time, another one flanked round and jumped over the wall. He was able to set off his suicide belt," Malian Captain Sidiki Diarra told reporters.
The bomber died and one Malian soldier was lightly wounded, he added. In Friday's motorbike suicide bomber attack, a Malian soldier was also injured.
Diarra described Saturday's bomber as a bearded Arab.
Since Gao and the UNESCO World Heritage city of Timbuktu were retaken last month, several Malian soldiers have been killed in landmine explosions on a main road leading north.
French and Malian officers say pockets of rebels are still in the bush and desert between major towns and pose a threat of hit-and-run guerrilla raids and bombings.
"We are in a dangerous zone... we can't be everywhere," a French officer told reporters, asking not to be named.
One local resident reported seeing a group of 10 armed Islamist fighters at Batel, just 10 km (6 miles) from Gao.
OPERATIONS IN NORTHEAST
The French, who have around 4,000 troops in Mali, are now focusing their offensive operations several hundred kilometres (miles) north of Gao in a hunt for the Islamist insurgents.
On Friday, French special forces paratroopers seized the airstrip and town of Tessalit, near the Algerian border.
From here, the French, aided by around 1,000 Chadian troops in the northeast Kidal region, are expected to conduct combat patrols into the Adrar des Ifoghas mountains.
In this rugged, sun-blasted range of rocky gullies and caves, the remaining Islamists are believed to have hideouts and supply depots and are also thought to be holding at least seven French hostages previously seized in the Sahel.
The U.S. and European governments back the French-led operation as a defence against Islamist jihadists threatening wider attacks, but rule out sending their own combat troops.
To accompany the military offensive, France and its allies are urging Mali authorities to open a national reconciliation dialogue that addresses the pro-autonomy grievances of northern communities like the Tuaregs, and to hold democratic elections.
Mali's interim President Dioncounda Traore, appointed after last year's military coup that plunged the West African state into chaos and led to the Islamist occupation of the north, has said he intends to hold elections by July 31.
But he faces splits within the divided Malian army, where rival units are still at loggerheads.