MBERA, Mauritania: Fears of ethnic reprisals by government troops in Mali have driven thousands of Arabs and Tuaregs in the country's north to abandon their homes and flee to Mauritania, undermining efforts to reunite their war-torn homeland.

At least 20,000 civilians have trekked westward across the dunes to the crowded Mbera refugee camp since mid-January when government forces reentered northern Mali on the coattails of a French ground and air campaign that swept Islamist rebels from the region.

The refugees joined 54,000 others who already fled to Mauritania when the rebels seized northern Mali in April 2012 and went on to impose a violent form of sharia law involving amputations and public whippings.

In the Mbera camp, where rows of plastic-covered shacks and makeshift tents stretch into the distance beneath the beating Saharan sun, many refugees have no plans to return to Mali despite the fall of the Islamists.

They say that for Arabs and Tuaregs - minority groups associated by many in Mali's black African majority with the rebels - life in the north is no longer safe.

"Innocent people were being arrested, killed and thrown down wells. It is inhuman," said Rissa Ag Cheibani, a local chief who fled from Goundam, near Mali's ancient desert town of Timbuktu. "What we are seeing is score settling."

Like many newly arrived refugees, Ag Cheibani said he had heard accounts of killings but not seen any himself.

While rights groups including Amnesty International and Human Rights Watch have documented dozens of extrajudicial killings of light-skinned Tuaregs and Arabs by Malian soldiers, the full extent of the problem is hard to assess.

Daily reports from Tuareg and Arab pastoralists of attacks by the Malian army on their desert settlements are impossible to verify, with many of the alleged victims simply disappearing.

In Timbuktu, Beitna Ould Cheikh, a nomadic Tuareg shepherd, said his brother, who plied the ancient salt route through the desert between Timbuktu and Taoudeni, vanished after being arrested by the Malian army in early February.

"He was arrested when they arrived in town... Since then, we have heard nothing," he said, with two nephews sat on his lap.

Mali's interim government, named following a military coup in March 2012, has said it will not tolerate ethnic reprisals and has recalled four soldiers to Bamako. The United Nations is also investigating alleged rights abuses.

The refugee exodus shows deep fissures in Malian society which may undermine international efforts to return stable government to the whole of the landlocked West African nation before elections in July.

The conflict in Mali began when MNLA Tuareg rebels launched an uprising early last year, driving Mali's army from the north as they sought to found a separate state they called Azawad.

Their revolt was swiftly hijacked by better-armed Islamists, including al Qaeda's North African wing, AQIM, an Arab-dominated group that has operated in northern Mali for a decade.

Many in the country's black African majority now blame the Tuareg and Arab groups for the violence which has threatened to tear apart their nation of 16 million people.

"We can no longer go to any markets because if we did, the black populations would attack us for the simple reason that 'red skins' are targeted," said Ahmed, a Tuareg refugee from Niafunke who arrived in Mbera with 20 others.

The flood of refugees to the camp seen in January has eased but every day scores still risk the journey of up to 200 km (125 miles) across Mali's harsh terrain. Arriving in Fassala, on the Malian side of the border, they wait under the harsh sun to be registered for the Mbera camp.

"It is worth it when you think of what would have happened if we had stayed at home," he said. "We no longer have any access to urban areas where we sell our cattle or buy goods."

The French-led offensive has killed hundreds of Islamists, leaving pockets of fighters scattered in the deserts and craggy, desolate Adrar des Ifoghas mountains bordering Algeria.

A reconciliation body has been set up, tasked with mending ties that have been strained over many decades by tensions between the southern-dominated government and the north but were taken to breaking point by last year's uprising.

France hopes to start withdrawing its forces from the end of April, handing over to African forces while pressing for the deployment of U.N. peacekeepers, with a mandate to prevent rights abuses.

For many in the north, that is reason for concern.

"We thought the problem would be resolved because France would twist the government's hand to listen to the north," said Ahmedou Ag Elboukhary, a tribal leader who fled from Lere. "Because without tribal dialogue between all the ethnic groups in the north, there will be no lasting peace."