By Adama Diarra and David Lewis

BAMAKO/DAKAR (Reuters) - Nine United Nations peacekeepers in Mali were killed when heavily armed gunmen on motorbikes ambushed their convoy on Friday, the deadliest attack yet on U.N. troops in the west African nation, the mission said.

The attack on the peacekeepers from Niger took place in the region of Gao and highlighted a sharp increase in strikes on foreign troops based in Mali to prevent the return of al Qaeda-linked Islamists who seized its northern desert region in 2012.

U.N. Secretary-General Ban Ki-moon was "shocked and outraged" by the attack and called for the perpetrators to be brought to justice, U.N. spokesman Stephane Dujarric said.

"They were targeting a convoy that included a fuel truck, knowing full well that an attack on a fuel truck would cause an even greater number of casualties, which adds to the horrendous nature of the crime," Dujarric said.

Dujarric said that 30 peacekeepers had been killed in Mali since the U.N. mission began in July last year, while another 90 had been wounded. He said Friday's attack took place between the northeastern towns of Menaka and Ansongo.

The U.N. mission said aircraft had been dispatched to secure the zone, which is near Mali's border with Niger.

The attack was condemned by the French and the Niger governments. Both reiterated their commitment to continue the fight against Islamist militants in northern Mali and the wider region.

"In any event, Niger is more than ever determined to fight tirelessly alongside other international forces against all forms of terrorism in the Sahelo-Sahara region," Niger's defense minister Karidio Mahamadou said in a statement.

ISLAMISTS

A security source said the peacekeepers were attacked in a dip in the road as it crossed a dry river bed.

U.N. peacekeepers have deployed across Mali's north in an effort to secure the country after the separatists and Islamists took advantage of the power vacuum created by a coup in the capital in 2012 to seize the northern regions.

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The Islamists were scattered by French forces early last year, elections have been held and rebels who distanced themselves from extremist groups have begun talks with the Bamako government.

But the peace process is moving slowly and Malian government troops abandoned most positions in the north earlier this year after clashes with the rebels.

Since then, Mali has called on the U.N. mission to deploy more of its mandated force of 12,000 men in the north, a zone that is awash with smugglers as well as various rebels.

However, diplomats say there is increasing concern that U.N. peacekeepers who are not trained or equipped for counter-insurgency warfare are over-exposed.

Ten Chadian peacekeepers were killed in Mali last month, prompting accusations from Chad that its troops were being neglected by the U.N. mission.

The spike in attacks on troops comes as France has redeployed some of its forces away from Mali as part of a plan to have 3,000 soldiers fighting extremists across the Sahara-Sahel band.

"There is a lot of concern at the moment," said one Bamako-based diplomat. "There have been concerns since the early French drawdowns. But the bad guys seem to have access to much more expertise and kit now too."

(Additional reporting by Michelle Nichols at the United Nations and Abdoulaye Massalaki in Niamey; Writing by David Lewis and Bate Felix in Dakar; Editing by Daniel Flynn and Gareth Jones)