Members of Al-Qaeda in the Islamic Maghreb (AQIM), ambushed the unit when they were 70 kilometres (45 miles) east of the mining town of Zouerat, the source said.

The official did not know how many people were injured in the attack -- the country's deadliest in three years -- but said 10 soldiers managed to return to base afterwards, while reinforcements had been sent to the area near the Moroccan border.

According to a local official seven people were injured in the ambush, including one seriously.

But another security source urged caution: "For now, we have to assume they are missing. We must wait for more information before we can talk about who has died or disappeared."

Opposition politicians blamed the ruling military junta for the attack, suggesting they were more interested in retaining the control they acquired during an August coup than defending their country.

"The army leaders left the borders defenceless against armed groups and brought the military to Nouakchott to defend their powers," opposition lawmaker Khalil Ould Teyeb said at a press conference.

His criticism drew a quick retort from pro-coup deputy Moustapha Ould Abeiderrahmane.

"Our country is victim of an ignoble attack and regardless of our political positions, there are some red lines we cannot cross," he said.

Last month a statement by AQIM appeared on the Internet condemning the August 6 military coup in Mauritania and urging the Mauritanian people "to prepare for war."

The suspected head of Al-Qaeda in Mauritania, El Khadim Ould Esseman, also urged the country's Muslims not to recognise the ruling military junta, calling it an "infidel regime."

Faced with a mounting threat from Islamic militants, security in Nouakchott was beefed up in the last three days, with checkpoints set up along the capital city's main crossroads.

Monday's ambush took place near Zouerat, "the iron capital," where the national industry and mining company, SNIM, employs a large number of foreign workers.

It also came as the African Union's security and peace commissioner, Ramdane Lamamra, was to meet coup leader General Mohamed Ould Abdel Aziz to try to broker an end to the country's constitutional crisis.

Mauritania, a vast desert country in northwest Africa, was shaken between December 2007 and February 2008 by three deadly attacks from extremists linked to Al-Qaeda which left seven people dead, including four French tourists.

At the beginning of April, a policemen and two extremists were also killed in a gun battle near Nouakchott.

More than 30 suspected Islamic militants are being held in Mauritania in connection with the attacks.

In 2005, an attack claimed by AQIM, formerly known as the Algerian-based Salafist Group for Preaching and Combat, on a military base in north-east Mauritania left 15 soldiers dead, two missing and 17 wounded.

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