Volunteers with Kurdish peshmerga forces clash with Islamic State militants in the town of Daquq, south of Kirkuk, September 30, 2014. REUTERS/Ako Rasheed

By Isabel Coles and Jonny Hogg

ARBIL, Iraq/MURSITPINAR, Turkey (Reuters) - Iraqi Kurdish troops drove Islamic State fighters from a strategic border crossing with Syria on Tuesday and won the support of members of a major Sunni tribe, in one of the biggest successes since U.S. forces began bombing the Islamists.

The victory, which could make it harder for militants to operate on both sides of the frontier, was also achieved with help from Kurds from the Syrian side of the frontier, a new sign of cooperation across the border.

Iraqi Kurdish Peshmerga fighters took control of the Rabia border crossing in a battle that began before dawn, an Iraqi Kurdish political source said.

"It's the most important strategic point for crossing," the source said.

The participation of Sunni tribal fighters in the battle against Islamic State could prove as important a development as the advance itself.

Members of the influential Shammar tribe, one of the largest in northwestern Iraq, joined the Kurds in the fighting, a tribal figure said.

"Rabia is completely liberated. All of the Shammar are with the Peshmerga and there is full cooperation between us," Abdullah Yawar, a leading member of the tribe, told Reuters.

He said the cooperation was the result of an agreement with the president of Iraq's Kurdish region after three months of negotiation to join forces against the "common enemy".

Gaining support from Sunni tribes, many of which either supported or acquiesced in Islamic State's June advance, would be a crucial objective for the Iraqi government and its regional and Western allies in the fight against the insurgents.

WINNING OVER SUNNI TRIBES

Winning over Sunni tribes was a central part of the strategy that helped the U.S. military defeat a precursor of Islamic State during the "surge" campaign of 2006-2007. Washington hopes the new Iraqi government can repeat it.

Rabia controls the main highway linking Syria to Mosul, the biggest city in northern Iraq, which Islamic State fighters captured in June at the start of a lightning advance through Iraq's Sunni Muslim north that jolted the Middle East.

Twelve Islamic State fighters' bodies lay on the border at the crossing after the battle, said Hemin Hawrami, head of the foreign relations department of the Kurdistan Democratic Party, one of the main Iraqi Kurdish parties, on Twitter.

Syrian Kurdish fighters said they had also joined the battle: "We are defending Rabia ... trying to coordinate action with the Peshmerga against Islamic State," said Saleh Muslim, head of the Syria-based Kurdish Democratic Union Party (PYD).

If Rabia can be held, its recapture is one of the biggest successes since U.S.-led forces started bombing Islamic State targets in Iraq in August.

It is one of two main border crossings between militant-held parts of the two countries, control of which has allowed Islamic State to declare a single Caliphate on both sides.

The ability to cross the frontier freely has been a major tactical advantage for Islamic State fighters on both sides. Fighters swept from Syria into northern Iraq in June and returned with heavy weapons seized from fleeing Iraqi government troops, which they have used to expand their territory in Syria.

Washington expanded the campaign to Syria last week in an effort to defeat the fighters who have swept through Sunni areas of both countries, killing prisoners, chasing out Kurds and ordering Shi'ites and non-Muslims to convert or die.

The United States hopes the strikes, conducted with help from European allies in Iraq and Arab air forces in Syria, will allow government and Kurdish forces in Iraq, and moderate Sunnis in Syria, to recapture territory.

But a wave of car bomb and mortar bomb attacks in Shi'ite areas of Baghdad, suspected to be the work of Islamic State fighters, were a reminder of risks. Iraqi police and medical sources said at least 35 people were killed.

Britain said its Tornado warplanes had launched their first attacks against Islamic State in Iraq since parliament approved combat operations last Friday, targeting a heavy weapons position that was endangering Kurdish forces and subsequently attacking an IS armed pick-up truck in the same area.

In Iraq, a coalition of Iraqi army, Shi'ite militia fighters and Kurdish troops known as Peshmerga have been slowly recapturing Sunni villages that had been under Islamic State control south of the Kurdish-held oil city of Kirkuk.

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