French, UN and Malian forces were engaged in a major operation aimed at preventing a resurgence of Islamist rebels in Mali, the French military said Thursday.

"We have engaged, with the Malian army and (UN mission) MINUSMA, in a large-scale operation" in the so-called Niger Loop, an area hugging a curve of the Niger River between Timbuktu and Gao, French general staff spokesman Colonel Gilles Jaron said.

"It is the first time we have seen forces of significant size working together," Jaron said.

About 1,500 troops were involved, including some 600 French, 600 Malians and 300 UN soldiers.

The goal of the mission -- dubbed "Hydra" -- was "to put pressure on any terrorist movements to avoid their resurgence," he said.

"This is one of those operations that are conducted regularly... to participate in the stabilisation of the country."

It was not clear exactly when the operation had started, but its announcement 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.

UN forces have been facing an upsurge in rocket attacks and bombings by militants ahead of nationwide elections next month in the troubled west African nation.

Jaron said such attacks were to be expected ahead of the first round of voting on November 24 and suggested they were not cause for serious concern.

"Every time, these are operations that are concentrated in one location, that are not long term and are based on terrorist acts... without having the ability to engage in lengthy combat," Jaron said.

"We know that not all the terrorist groups present... in Mali have been eliminated," he said.

A French-led offensive in January drove Islamist groups linked to Al-Qaeda out of cities of northern Mali -- including Gao, Kidal and Timbuktu -- that they occupied in the wake of a coup in Bamako last year.

But the rebels have taken to bases in the surrounding mountains and launched strikes on the French and peacekeeping forces.

UN special representative to Mali Bert Koenders said last week that recent attacks had been "an important wake-up call" over security.

France has 3,000 troops in its former colony but Paris plans to draw down the force to 1,000 servicemen by the end of January.

The UN peacekeeping force is eventually expected to include about 12,600 troops.

Concerns have also been raised about divisions within the Malian army, with Amnesty International on Wednesday saying the military was carrying out a purge of some soldiers involved in protests at a barracks outside Bamako last month.

The rights group's French branch said in a statement that four soldiers' bodies had been discovered near Bamako in early October and several other troops, including a colonel, were missing and feared dead.