* Death of preachers will complicate ties between neighbours
* Mauritania key to regional fight against al Qaeda
* Must also accommodate interests of own Islamist groups
NOUAKCHOTT, Sept 10 (Reuters) - Mauritania called the killing 16 Muslim preachers by Mali's army a barbaric act carried out in cold blood and demanded to be part of any investigation into the incident.
The killing at the weekend of eight Mauritanians and eight Malians creates a diplomatic crisis at a time when regional leaders are struggling to agree on a strategy to regain control of Mali's north from Islamists, including al Qaeda fighters.
"The government ... expresses its indignation at this unspeakable criminal act, committed in cold blood, without any warning ... against preachers who had no weapons but their faith and were coming with a message of peace," a statement carried by state news agency AMI said late on Sunday.
Mauritania's statement accused Malian army units of a "barbaric massacre" of peaceful Muslim preachers travelling to Bamako. Religious leaders in the Malian capital said the preachers were going to a conference there.
Those killed are believed to be from the Dawa movement, which preaches a fundamentalist ideology but does not advocate violence. Iyad ag Ghali, one of leaders of the Islamist groups in northern Mali, is a former member of Dawa and he has been criticised by many of its members for the rebellion.
A military officer said soldiers had opened fire pre-emptively to ward off a possible ambush. The Malian government has ordered an investigation.
One of the Islamist groups in Mali's north said the killing was a declaration of war.
"(The government) demands the swift opening of an independent investigation in order to establish the facts of this odious crime, identify those responsible and bring them to justice ... (we) would like to be part of this investigation," the Mauritanian statement said.
With one of West Africa's more effective armies, Mauritania has been one of the West's strongest allies in the fight against al Qaeda in the region and is expected to be part of any joint effort to wrest control of the north from the Islamists.
However, as an Islamic republic straddling black and Arab Africa, it must also juggle the interests of influential and often critical domestic Islamist groups.
Dozens of people gathered on Monday morning outside the Mauritanian president's office condemning the incident.