France's defence minister has confirmed that French forces are engaged in heavy fighting in northern Mali, as suicide car bombing killed six government allies in the city of Kidal.

The bomber carried out the attack on Tuesday evening at a checkpoint at an entrance to Kidal, Ag Alghabas Intalla, a leader of the Islamic Movement of Azawad, or MIA, said by phone from Kidal.

He said he counted six dead and others wounded. The MIA group is fighting with the Malian army and French troops against armed rebels.

Responsibility for the suicide attack has not been claimed, but it is suspected to be the work of fighters of the Movement for Oneness and Jihad in West Africa, or MUJAO.

"The suicide attack targeted the checkpoint on the eastern side of Kidal which is manned by the MNLA (National Movement for the Liberation of Azawad)," a French military official told AFP news agency in Gao, the main city in the north.

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

Several other officials confirmed the blast was an attack following an initial report that it may have been a controlled explosion of ammunition seized from the rebels.

'Very violent fighting'

With no let-up in violence, Jean-Yves Le Drian, France's defence minister, announced that French troops were involved in "very violent fighting" in the Adrar des Ifoghas mountains of northeastern Mali.

Le Drian said that it was too early to talk about a quick French pullout from Mali, despite the growing cost of the intervention.

The French intervention in Mali has cost more than $133m since it started on January 11, he said on France's RTL radio.

"We are now at the heart of the conflict," in protracted fighting against rebels in the Ifoghas mountains, Le Drian said.

While some expected the 4,000-strong French force to pull out next month, Le Drian said he could not talk about a quick withdrawal while the mountain fighting goes on.

Soldiers from Chad and a few other African countries have joined the French-led operation to help Mali's weak military push back the al-Qaeda-linked groups who had imposed harsh rule on northern Mali and started moving southward towards the capital, Bamako, last month.

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.

A clash in the area killed 23 soldiers from neighbouring Chad last week, according to French President Francois Hollande, who expressed condolences to his Chadian counterpart.

UN official's appeal

Against this backdrop, a senior UN humanitarian affairs official said in New York on Tuesday that as security improves in Mali, the world must seize the moment to deliver much-needed humanitarian aid.

John Ging, who just visited Mali, said the country's northern region is stabilising but needs help reopening schools, markets and health clinics.

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

Even before fighting erupted last year among government forces, Tuareg rebels and al-Qaeda-linked fighters, Ging said, Mali was suffering from the severe food crisis that has hit Africa's arid Sahel region.

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

The groups conquered much of northern Mali after a military coup in Bamako, aided by al-Qaeda's North Africa wing.

In Timbuktu, they imposed strict Islamic law and forced thousands to flee; others were tortured and executed.

But the French-led intervention in January brought the fighters to quit the northern cities of Timbuktu, Gao and

Kidal and retreat to mountainous hideouts near the Algeria border.