* Despite loss of leader, young Sunni tribesmen prime for battle

* Iraq struggling to overcome sectarian tensions

* Already wounded, teenager vows victory over jihadists

By Michael Georgy

DEBAGA, Iraq, Oct 25 (Reuters) - Still limping four months after an Islamic State fighter fired a bullet into his leg, teenager Mustafa Muhammed says he and his fellow Sunni tribesmen are ready to join an offensive against the militants in their stronghold of Mosul.

Sitting in a truck in a camp where they live as refugees from the jihadists, dozens of young men - part of a force of hundreds of tribesmen - will soon head to the frontlines.

There they will face an enemy which seized control of their lands around Mosul two years ago, and whose fighters they know as fearless and highly unpredictable.

"Daesh is never scared. They want to die because they believe they will be martyrs," said Muhammed. "That's why they come to battle strapped with suicide belts."

Daesh is the Arabic acronym for Islamic State, which controls large parts of neighbouring Syria and swept through northern and western Iraq in 2014.

Since then Iraq's Shi'ite-led government has tried to encourage Sunni tribesmen to join the fight against the ultra-hardline Sunni group. But deep distrust between the country's two dominant sects, which flared into civil war after the U.S.-led invasion in 2003, prevented any meaningful cooperation.

Sunnis, who dominated Iraq under Saddam Hussein, accuse Shi'ite leaders of marginalising them through sectarian policies, allegations Baghdad's Shi'ite-led government denies.

The scene at the Debaga camp suggests that a shared hatred for Islamic State means that - at least for now - Sunnis are ready to overcome sectarian divisions and join the government's fight against a common enemy.



TRIBAL LOYALTIES

That may not last for long after the Mosul campaign, expected to be the most complex military operation in Iraq since a U.S.-led invasion toppled Saddam in 2003.

The loyalty of the teenagers in Debaga is expressed not for their country, a complex and combustible mix of Shi'ites, Sunnis, Kurds, Yazidis and Christians, but for a Sunni tribal leader named Sheikh Faris Abdullah.

He and other Sunni tribesmen fought al Qaeda during the U.S. occupation of Iraq, under an American-backed initiative that proved highly successful, all but wiping out al Qaeda in Iraq between 2007 and 2009.

But the militants regrouped under the banner of Islamic State, whose leader Abu Bakr al-Baghdadi declared a caliphate from the pulpit of a Mosul mosque in 2014. When the group began ordering executions and beheadings of opponents, Faris established a new Sunni fighting force to oppose it.

His fighters took on the group, hoping to recapture towns and villages, and continuing even after an Islamic State sniper killed Faris during an operation in a village in the Mosul area.

"We will win the war against Daesh. After that Sunnis should rule themselves in their own region," said Alaa Ahmed, one of the fighters, in military fatigues.

As flies swarmed around the displaced and young children walked barefoot on sand and gravel, some of the fighters smoked cigarettes to pass the time. Others stood in chatting in alleyways.

Debaga, a sprawling camp of mainly prefabricated houses with corrugated iron roofs, lies on the outskirts of Erbil, about 75 km (45 miles) east of Mosul.

Sheikh al-Muqdad Abdullah, Faris's son, now leads the force and is optimistic about the chances of defeating Islamic State in Mosul and even building a new sense of unity in Iraq.

He said he was encouraged by recent gains made by the Iraqi army and Kurdish fighters against the jihadists, which intelligence officials say have rigged bombs across Mosul, Iraq's second largest city.

The fighters say they have received basic military training from the U.S.-led coalition, including how to handle AK-47 assault rifles and identify improvised bombs.

The driver of the truck carrying tribesmen held up a phone and showed a photograph of himself with an AK-47 standing over the torched corpse of an Islamic State fighter.

"We will win and there will be all kinds of reforms so that Sunnis have a voice," he said.

Asked how he had prepared his young tribesmen for Islamic State tactics, he said: "I tell them that a box of cigarettes just like this one could be placed somewhere. As soon as you touch it, it could blow up and kill you."

(Reporting by Michael Georgy; Editing by Dominic Evans) ((michael.georgy@thomsonreuters.com;)(M)(+2010 271 00098)(O)(+20 225783290; Reuters Messaging: michael.georgy.thomsonreuters.com@reuters.net))