As many as 12 fighters from the Al-Qaeda in the Islamic Maghreb (AQIM) group have been killed in a counter-terrorism operation by French forces in Mali who used drones to track down the Jihadists, France's defence minister said Thursday.

The offensive took place Tuesday night in northeastern Mali, Jean-Yves Drian said.

Separately, a UN source confirmed that "a very large French operation" in the area had taken place.

The AQIM fighters were spotted by French forces operating the US-made Reaper drones, Drian said, confirming a report which had appeared in the daily Le Figaro.

"These are the drones which the French army now has in place which allows us to identify these groups," said Drian.

Ministerial sources said the those killed belonged to the AQIM group, operating in a mountainous region of northeastern Mali.

According to the same source, use of the drones -- as well as Mirage 2000 fighter jets and military helicopters -- underlined the increasingly sophisticated weaponry now being employed by the French military.

The Reaper drones are based in Niamey, the capital of Niger, which borders Mali to the south. No ground forces were used in the operation, said Drian.

The minister added that, despite the success of the latest operation, French forces in the region would remain vigilant.

A source from MINUSMA, the United Nations peacekeeping mission in Mali, told AFP that the French assault "has led to the neutralisation of a dozen terrorists".

The UN promised further details of the operation later.

A Malian army source informed AFP that "Islamists, notably from Libya, have reorganised themselves on the ground".

"They set up two bases in the north-east that the French from (Operation) Serval... destroyed on Tuesday night. At least 11 terrorists were killed," said the same source.

Mali was thrown into chaos in 2012 when Tuareg separatist rebels launched an offensive in the northern desert helped by Islamist militants linked to Al-Qaeda, after the country's president was toppled in a coup.

The Islamists took control of northern Mali, ruling it under a brutal version of Islamic law until former colonial ruler France sent in troops to flush them out in January 2013.

UN peacekeepers took over security in July last year from the Pan-African AFISMA military mission, which had been supporting the French troops.

France is winding down its deployment from a peak of around 5,000 soldiers but is to keep 1,000 troops in Mali beyond the spring.