A major anti-Islamist military offensive in Mali by French, Malian and UN troops has highlighted worries over the possibility of renewed attacks in the run-up to nationwide polls, say analysts.

The troubled west African nation and its allies launched Operation Hydra on Sunday after being taken aback by a recent upsurge in violence in the north, according to Jean-Herve Jezequel, an analyst in Dakar for the International Crisis Group (ICG) think tank.

About 1,500 troops are involved, including some 600 French, 600 Malians and 300 UN soldiers, and the operation is expected to last "at least 10 days", according to a Malian military source.

French leader Francois Hollande said Friday Hydra marked the first time that such a large deployment from each of the allies had worked together in Mali to fight "terrorism" which he said had become entrenched in the north and in other parts of Africa's Sahel region.

France launched a military intervention in January to oust armed groups linked to Al-Qaeda that had occupied the vast desert north for the previous nine months, meting out beatings and executions as part of a brutal regime of Islamic sharia law.

Hollande said the infrastructure of terrorism in Mali had been damaged but "not defeated" by the operation.

Under heavy attack in the cities of Gao, Timbuktu and Kidal, the Islamists retreated into the desert and the cover of the Ifoghas mountains.

But the fragile peace did not last, and on September 28 the militants resumed their deadly insurgency, killing a dozen civilians and UN peacekeepers across the north in three weeks.

France's announcement of the launch of Hydra came after two Chadian UN peacekeepers and a civilian were killed in northern Mali on Wednesday.

Al-Qaeda-linked militants said they were behind the attack on a United Nations checkpoint in the far northern town of Tessalit.

The UN Security Council strongly condemned the attack, which followed an urgent request by the UN mission in Mali for more troops.

The upsurge in violence comes with just a month to go until parliamentary elections which are supposed to mark the completion of Mali's transition back to democracy following a military coup in March last year that precipitated the fall of the north to the Islamists.

A presidential election in August which saw former prime minister Ibrahim Boubacar Keita voted in as the first post-conflict head of state was staged without incident, in all likelihood because the recently-defeated Islamists had not had time to regroup.

The ICG's Jezequel believes however that elections such as November's legislative polls could be ideal targets for the type of attacks carried out by armed Islamist groups.

France had planned to reduce its 3,000-strong force to 1,000 by the new year but the fresh violence has forced Paris to rethink its timetable.

Some 2,000 troops will now remain until the end of December, with the withdrawal of a further 1,000 to be completed by the end of January 2014.

The UN peacekeeping force, known as MINUSMA, is eventually expected to comprise about 12,600 troops and police but the prospect of France's departure from its former colony -- even deferred -- is still worrying the Malian army.

"MINUSMA alone cannot do the job," said a Malian soldier in Gao, the largest city in the north and the site of a rocket attack by Islamists on October 7.

"The French army is needed in the north. The Islamists have begun to seriously reorganise."

Another Malian military source told AFP the French army had helped with the recent arrest of terror suspects in Gao -- evidence, he said, "that the danger is ever-present".