France's defence minister has said that French troops are involved in "very violent fighting" in the mountains of northern Mali and that it is too early to talk about a quick pullout from the West African country, despite the growing cost of the intervention.

The fighting against rebels in the Adrar des Ifoghas mountains has been going on for days.

A clash in the area killed 23 soldiers from neighbouring Chad on Friday, according to a letter from French President Francois Hollande expressing condolences to his Chadian counterpart.

Soldiers from Chad and a few other African countries have joined the French-led operation to help Mali's weak military push back rebels who had imposed harsh rule on northern Mali and started moving toward the capital last month.

Meanwhile, a suicide bomber in northern Mali drove a car bomb on Tuesday at a checkpoint in the city of Kidal manned by Tuareg rebels supporting the French-led offensive, killing at least six people.

"The suicide attack targeted the checkpoint on the eastern side of Kidal which is manned by the MNLA," a French official said in Gao, the main city in northern Mali.

The attacker struck a checkpost manned by Tuareg separatists supporting the French-led military offensive against Islamist insurgents.

A hospital source told AFP news agency there were seven dead including the bomber, with another 11 wounded.

Increasing costs

French Defence Minister Jean-Yves Le Drian said on France's RTL radio on Tuesday that the French intervention in Mali had cost more than $133 million since it started January 11.

In the first weeks of the campaign, French and Malian forces easily took back cities in northern Mali. But the fighting is rougher now that it has reached more remote terrain in the mountains of the southern Sahara.

"We are now at the heart of the conflict," in protracted fighting in the Adrar des Ifoghas mountains, Le Drian said. While some have suggested starting a pullout of the 4,000-strong French force next month, Le Drian said he couldn't talk about a quick withdrawal while the mountain fighting goes on.

Hollande's letter to Chadian President Idriss Deby said the deaths of Chadian soldiers "illustrate the dangers of this mission." It gave no details. The Chadian army had initially said that 13 soldiers and 65 Islamic extremist rebels were killed in the fighting Friday.

At the United Nations in New York, a top UN humanitarian official said on Tuesday that as security improves in Mali, the world must seize the moment to deliver much-needed humanitarian aid.

John Ging, a senior humanitarian affairs official who just visited Mali, said that country's northern region is stabilising but needs help re-opening schools, markets and health clinics.

The UN is appealing for $373 million in aid, but has only received $17 million.

Even before fighting erupted last year among government forces, Taureg rebels and radical Islamists, Ging said Mali was suffering from the severe food crisis that has hit Africa's arid Sahel region.

Ging said more than 430,000 Malians have been displaced.