Black-robed Islamic extremists armed with AK-47 automatic rifles snuck into the city of Gao in canoes Sunday to launch a surprise attack on the Malian army in the most populous city in northern Mali, two weeks after French and Malian troops ousted the jihadists.

The combat started at about 2 p.m. in downtown Gao and the fighting was continuing as night fell. Later the sound of gunfire was replaced by the clattering of French military helicopters overhead.

The attack in Gao shows the Islamic fighters, many of them well-armed and with combat experience, are determined and daring and it foreshadows a protracted campaign by France and other nations to restore government control in this vast Saharan nation in northwest Africa.

The Islamic radicals fought against the Malian army throughout the afternoon and were seen roaming the streets and on rooftops in the centre of Gao, which has a population of 90,000. Gunfire echoed across the city.

French soldiers had taken control of Gao on Jan. 26 but are now under attack. (Jerome Delay/Associated Press)

Families hid in their homes. One family handed plastic cups of water through the locked iron gate to others hiding on their patio. Piles of onions lay unattended where market women fled when the Islamists arrived. There were no signs of civilian casualties.

The fighting appeared to centre near the police headquarters, where Malian soldiers with rocket propelled grenades traded fire with the combatants believed to be from the Movement for Oneness and Jihad in West Africa, or MUJAO. The only sound in the city was gunfire and the bleating of goats. Soldiers were positioned at every corner in the neighbourhood of mud-walled buildings.

Ever since French forces took Gao on Jan. 26, the Islamic rebels had clashed with security forces on the city's outskirts. This was the first time they succeeded in entering the strategic city.

The Islamic fighters used canoes to cross the Niger River to penetrate Gao, according to French Gen. Bernard Barera, who cited Malian officials.

The Islamic radicals had already tried to spread violence into Gao. On Saturday night, a suicide bomber detonated himself at a checkpoint at the entrance to the city, killing himself and injuring one Malian soldier. An earlier suicide bomber on a motorcycle also blew himself up at the same security spot on Friday, killing only himself.

Besides Gao, French and Malian forces have also retaken the fabled city of Timbuktu and other northern towns, pushing the Islamic extremists back into the desert, where they pose a constant threat to Malian and allied forces. But the Islamic fighters made strategic retreats and are dug into desert hideouts, from where they are expected to continue challenging the control of the cities by French, Malian and allied forces.

Several African nations have contributed troops to battle the extremists, who imposed their harsh version of Islamic Shariah law when they controlled the northern cities.

The armed Islamic fighters seized the northern half of Mali in April 2012, sending poorly disciplined and equipped Malian forces retreating in disarray. France launched its military intervention in its former colony on Jan. 11 when the Islamic radicals, many of whom had fought for ex-Libyan leader Moammar Gadhafi, began encroaching on the south, threatening the capital Bamako which lies deep in southern Mali, 1,200 kilometres from Gao.

France has said that it wants to hand over responsibility to the Malian military and other African nations who have contributed troops and has raised with the United Nations Security Council the possibility of establishing a UN peacekeeping operation in Mali. .