Pressure has been growing on Malian troops over summary killings and rights abuses in a French-led assault on al-Qaeda-linked groups.
Meanwhile, African forces were moving towards Mali's centre on Thursday.
The first troops from a UN-mandated African force aimed at replacing the French mission have "already started to move towards central towns", foreign minister Laurent Fabius said in Paris on Wednesday, adding that the force had deployed "far quicker than envisaged".
He said 1 000 troops from West African countries and Chad had already arrived in Mali, which has been split in two since April. "The African force is deploying much faster than expected," Fabius said. "Obviously that poses a number of logistical difficulties but I have to say that I have seen a very big effort by our African friends."
A Malian defence official said that 160 soldiers from Burkina Faso had arrived in Markala, 270km north of the capital Bamako, to "take up the baton from the French" guarding a strategic bridge on the Niger River.
"They are already in place and could then go on to Niono and Diabaly," two towns farther north, the source said, adding: "After the French, it will be the Africans who are on the ground".
The UN has authorised the deployment of a 3 300-strong force under the auspices of 15-nation West African bloc Economic Community of West African States. But the involvement of Chad, which has committed up to 2 000 troops, means the force could now be much bigger.
Human rights abuses
Nearly two weeks after France swept to Mali's aid to stop an extremist advance towards Bamako, reports emerged of atrocities committed by Malian soldiers and growing fears of attacks among light-skinned ethnic communities.
The majority of the al-Qaeda-linked rebels being hunted by the armies are either Tuaregs or Arabs. The International Federation of Human Rights Leagues said that in the central town of Sevare at least 11 people were executed in a military camp near the bus station and the town's hospital, citing evidence gathered by local researchers.
Credible reports also pointed to around 20 other people having been executed in the same area and the bodies having been dumped in wells or otherwise disposed of, the organisation said. At Niono, also in the centre of the country, two Malian Tuaregs were executed by Malian soldiers, according to the International Federation of Human Rights.
The organisation called for an immediate independent inquiry to "determine the scale of the abuses and to punish the perpetrators". The rights group Human Rights Watch said its investigators had spoken to witnesses who saw the executions of two Tuareg men in the village of Siribala, near Niono.
The group also said witnesses had reported "credible information" of soldiers sexually abusing women in a village near Sevare, and called on the government to urgently investigate these incidents.
'We cannot accept any rights violations'
French defence minister Jean-Yves Le Drian urged extreme "vigilance" against any abuses, saying the "honour of the [Malian] troops is at stake". "We cannot accept any rights violations. The international community will face a very serious situation if [the intervention force] is identified with abuses," Fabius added.
Le Drian said France had run up a bill of €30-million in 12 days in Mali, but added that it would not make a serious hole in the defence budget, which included about €630-million for external operations.
France said it had already deployed 2 300 soldiers in Mali, its former colony, whose poorly trained and equipped force has been overwhelmed by Islamist rebels occupying the vast arid north and seeking to push south. Malian army chief General Ibrahima Dahirou Dembele promised that any soldier involved in abuses would be brought to book.
"One mustn't get confused. Every white skin is not a terrorist or a jihadist and among the enemy which attacked our different position were many black skins. We are among brothers, whether one is black or white."
International moves to aid the French-led operation have accelerated, with the US military airlifting French troops and equipment from France into Mali. Italy, Germany, Belgium, Britain, Canada, Denmark, the Netherlands, Spain and the United Arab Emirates are also providing transport planes or helicopters required to help move the African and French troops around Mali's vast expanses.
?Mali's year-old crisis began when Tuaregs returning from fighting for slain dictator Moamer Kadhafi in Libya, battle-hardened and with a massive arsenal, took up a decades-old rebellion for independence of the north, which they call Azawad.
They allied with hardline extremists and seized the key towns of Kidal, Gao and Timbuktu in a matter of days. The extremists later broke with their Tuareg allies, and with firm control of the north, implemented an extreme form of Islamic law.
Nigerian President Goodluck Jonathan, who has pledged to send 900 soldiers to Mali, meanwhile said that local extremist group Boko Haram was linking up with al-Qaeda's north African branch operating in northern Mali and other north African nations.
If not contained, Boko Haram would threaten "not only Nigeria, but west Africa, central Africa and of course north Africa", he told CNN. – Sapa-AFP