Islamic extremists have taken the offensive around Gao in north Mali, wounding two civilians Tuesday when they blew up a bridge, a day after shelling the town and injuring a soldier.

"Early this Tuesday, Islamists dynamited one of two small bridges near ... Bentia, about 50 kilometres (31 miles) from the border with Niger, leaving two civilians wounded," said Ibrahim Cisse, a local councillor for the Gao region.

Cisse said that the assailants "wearing turbans" arrived by motorbike at the bridge that crosses the Niger River at Bentia, then destroyed it.

"In this place... there are two small bridges. The aim of the Islamists was to blow up both bridges, but fortunately only the old one was badly damaged," a police source in Gao said.

"The new bridge, which is the most frequently used, sustained only very slight damage," the source added.

Malian soldiers were sent to the spot, along with French troops deployed in the desert north of the vast country, "to avoid other acts of sabotage" by armed extremists, the police source said.

Islamist groups linked to Al-Qaeda were driven out of Gao, the largest town in the north, along with Kidal and Timbuktu, after they occupied all three in the wake of a coup in Bamako last year.

France keeps 3,000 troops in its former colony out of a contingent it despatched last January to help end the Islamist occupation, but Paris plans to draw down the force to 1,000 men by the end of the year.

The bridges at Bentia were attacked a day after armed Islamists fired shells on Gao itself, wounding a Malian soldier, and come 10 days after a suicide attack in Timbuktu killed two civilians and four bombers, as well as wounding seven Malian soldiers.

A spokesman for an Al-Qaeda splinter group, the Movement for Oneness and Jihad in West Africa (MUJAO), on Tuesday claimed responsibility for both the Gao attacks and warned of further operations, in a message to AFP.

"In the name of all mujahedeen, we claim the attacks against the evil-doers in Gao and the attack on the bridge (that served) to transport the enemies of Islam into the land of Islam," Abou Walid Sahraoui said, warning that "attacks on the enemies of Islam will continue".

Responsibility for the Timbuktu attack was claimed by Al-Qaeda in the Islamic Maghreb (AQIM), which was founded in Algeria and operates across the Sahel region south of the Sahara.

On Monday night, Mali's Defence Minister Soumeylou Boubeye Maiga denounced the Gao shelling by "presumed terrorists" and said that "three of the four devices" fired had exploded.

Maiga said he wanted to "reassure the population that in coordination with our partners in Serval (the French operation) and MINUSMA (the UN's African military force in Mali), our deployment has been strengthened".

He urged the population "to remain calm and above all to share information with personnel of the armed forces and security forces in order to help them track down the enemy in all its forms".

Last year Mali was upended by a military coup that toppled the elected president and was followed by a Tuareg separatist insurgency.

Islamist fighters then took the upper hand in northern towns and imposed strict Sharia law, marked by numerous violations of human rights, until they were ousted.

French and African military support for Mali's army drove the Islamists into desert hideouts, notably the Ifhogas mountains in the northeast, from which they still operate.