By Isabel Coles

ARBIL, Iraq (Reuters) - Kurdish forces in northern Iraq are drawing up plans to break Islamic State's siege of Sinjar mountain, where hundreds of minority Yazidis remain stranded months after fleeing their homes.

Seeking to regain territory and repair pride in his military forces, Masoud Barzani, president of Iraq's semi-autonomous Kurdish region, is overseeing efforts to retake the mountain, senior party members said.

Islamic State attacked the Sinjar area in August, sending thousands of Yazidis fleeing up the mountain, a craggy strip some 40 miles (65 km) long.

Hundreds of Yazidis were executed, Iraqi officials and witnesses said, by Islamic State militants who see the adherents of an ancient faith derived from Zoroastrianism as devil-worshippers. A senior U.N. rights official said the onslaught looked like “attempted genocide”.

Kurdish peshmerga forces have regained between 65 and 75 percent of the ground lost to Islamic State in the area since the U.S. began a campaign of air strikes in August, said Halgurd Hikmat, spokesman for the Kurdish Peshmerga Ministry.

But Sinjar's awkward geography -- out on a limb to the west, has made it difficult to penetrate.

"Our priority now is Sinjar," said Hikmat. "A plan will be in place within the coming days."

The strategy was to cut off an Islamic State supply route between Mosul and Syria which runs along the southern foot of the mountain, Hikmat said. He did not elaborate.

Controlling Sinjar would put the peshmerga on three sides of Mosul, the largest city under Islamic State control in northern Iraq, and allow them to gain positions for any future offensive to retake the city and nearby areas which have been the target of Iraqi and U.S. air strikes.

"After that, we must coordinate with Baghdad and the coalition (of Western and Gulf Arab states) to get Islamic State out of Mosul," said Hikmat.

Mosul has become the focus of the government's military efforts because of both its size and its symbolic status after Islamic State leader Abu Bakr al-Baghdadi delivered a public speech at the Grand Mosque there in July,

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Baghdadi, who sees himself as 'caliph' of an Islamic state he has declared in parts of Iraq and Syria, told his fighters they were victorious after years of patience and holy struggle.

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Estimates of the number of people still stranded on Sinjar mountain -- part of disputed land claimed by both the Kurds and Baghdad -- vary from 10,000 to fewer than 1,000.

Last month, the peshmerga recaptured the town of Rabia, taking control of a crossing point into Syria and moving closer to Sinjar.

Further advances may not come easily. An intelligence officer in the Rabia area said it would take between two to three weeks to drive Islamic State out of the villages north of Sinjar mountain, because many Arab residents either supported Islamic State, or opposed Kurdish encroachment.

A Yazidi fighter told Reuters by telephone from the mountain that he was one of around 1,500 volunteers there, in addition to a contingent of around 150 peshmerga. Hikmat put the number of peshmerga at 2,000.

"We have received several (types of) small arms from the Kurds, including AK-47s, sniper rifles, mortars and light machine guns," said the volunteer fighter, who gave his name as Barakat.

Politically, Sinjar has been damaging for the Kurdistan Democratic Party (KDP) -- one of two main Kurdish power centers -- whose forces were responsible for protecting the area.

Since 2003, the KDP has courted and co-opted minorities in the disputed areas along its southern border by providing them with jobs, security and services in exchange for loyalty.

"The KDP and the Yazidis are inseparable parts of the same body, and that also applies to the KDP and Sinjar," Barzani, who is also head of the KDP as well as commander-in-chief of the peshmerga, told Yazidi members of his last month.

Rival parties have capitalized on the loss of Sinjar to score political points against the KDP.

The Syrian Kurdish YPG militia, which crossed into Iraq and saved thousands of Yazidis over the summer has won favor with the minority, many of whom felt betrayed by the peshmerga.

Thousands of Yazidis have since been trained by the YPG in Syria, and a small number of guerrillas remain on the mountain, according to fighters there.

Peshmerga spokesman Hikmat said the YPG would not participate in the Sinjar offensive, but that the peshmerga would coordinate with them to protect the area afterwards.

(Additional reporting by Saif Sameer; Editing by Michael Georgy and Robin Pomeroy)