Travellers driving from Niamey, Niger, line up to be searched at the entrance to Gao, in northern Mali, on Tuesday. Soldiers from Niger and Mali patrolled downtown Gao on foot on Tuesday, combing the sand footpaths through empty market stalls to prevent radical Islamic fighters from returning to the embattled city.

Gao - Mali risks descending into “catastrophic” violence, the United Nations rights chief warned on Tuesday, as tensions swept the country after a string of attacks by Islamist rebels on French-led forces.

After four days of suicide bombings and guerrilla fighting in the northern city of Gao, fears of fresh attacks were high following a call from Al-Qaeda in the Arabian Peninsula (AQAP) - which US officials have labelled al-Qaeda's most dangerous franchise - for a holy war in Mali.

UN rights chief Navi Pillay warned a second kind of violence also threatened the country - reprisal attacks by the army and black majority on light-skinned Tuaregs and Arabs accused of supporting the rebel groups that have plunged Mali into crisis.

“As the situation evolves, attacks and reprisals risk driving Mali into a catastrophic spiral of violence,” Pillay told the UN Security Council.

“Protection of human rights is key to stabilising the situation.”

Pillay said human rights investigators from her department had started arriving in the Malian capital last week, and called on all sides in the conflict to refrain from revenge attacks.

Rights groups have accused the Malian army of killing suspected rebel supporters and dumping their bodies in wells.

Tuaregs and Arabs have also come under attack from their black neighbours in northern towns such as Timbuktu, where looting broke out after French-led forces reclaimed the city and a mob tried to lynch an alleged Islamist supporter.

A grave containing several Arabs' bodies was recently discovered in Timbuktu.

With fears of reprisal attacks high, many Arabs and Tuaregs have fled.

In all, the crisis has caused 377 000 people to flee their homes, including 150 000 who have sought refuge across Mali's borders, according to the UN.

Mali imploded after a March 22 coup by soldiers who blamed the government for the army's humiliation at the hands of north African Tuareg rebels, who have long complained of being marginalised by Bamako.

With the capital in disarray, al-Qaeda-linked fighters hijacked the Tuareg rebellion and took control of the north.

Analysts say the crisis has been fuelled by a complex interplay of internal tensions and international factors, including al-Qaeda's call to jihad.

Those concerns were underlined on Tuesday when AQAP, al-Qaeda's Yemen-based branch, condemned France's month-old intervention as a “crusader campaign against Islam” and called all Muslims to join a holy war against it.

“Supporting the Muslims in Mali is a duty for every capable Muslim with life and money, everyone according to their ability,” AQAP's Sharia Committee said in a statement reported by the SITE Intelligence agency.

Around 90 percent of Malians are Muslim, but the Islamists' hardline ideology is not broadly accepted.

AQAP said jihad is “more obligatory on the people who are closer” to the fight and that “helping the disbelievers against Muslims in any form is apostasy”, said US-based SITE, which monitors extremist Internet forums.

The statements were an apparent reference to north African countries, notably Algeria, where Islamist gunmen attacked a gas field after the government agreed to let French warplanes use Algerian airspace, unleashing a hostage crisis that left 37 foreigners dead.

Al-Qaeda in the Islamic Maghreb, al-Qaeda's North African branch, is one of the groups that seized control of northern Mali for 10 months in the wake of a March coup, along with the Movement for Oneness and Jihad in West Africa (MUJAO) and and Ansar Dine (Defenders of the Faith). - Sapa-AFP