Dressed in green and carrying a heavy rifle, Yazan al-Jabouri paced along the front line on the fringes of the northern city of Tikrit, anxiously awaiting a plan of attack.

For months, al-Jabouri lived in fear of crossing paths with a fighter from the Islamic State group. He had refused to pledge loyalty to the extremist group, and letters had been left on his doorstep threatening to cut his head off. His wife and children also were threatened.

"I wanted nothing to do with Daesh," he said, using the Arabic acronym for the group, also known as ISIS and ISIL. He said 156 homes of his kinsmen had been destroyed and 16 of his relatives had been killed.

Despite the threats and the violence, he felt he had to take up arms against the extremists, many of whom are fellow Sunnis.

"I can't sit quietly while they terrorize my people," he said.

Al-Jabouri, a member of one of Iraq's most prominent tribes, is among about 150 Sunnis fighting in a tense alliance alongside the Iraqi army and tens of thousands of Shiite militiamen to try to recapture Tikrit from the Islamic State militants.

While small in number, the Sunni brigade is viewed as a crucial component against the extremists, who captured territory mainly in Iraq's predominantly Sunni provinces last year.

They receive weapons and supplies from the Iraqi government, much of which al-Jabouri said originates from Iran, which is providing enormous support to Iraqi security forces, training, advising and equipping fighters on the battlefield.

The offensive to retake Saddam Hussein's hometown of Tikrit began March 2. The city is one of the largest held by the Islamic State militants on the road connected Baghdad and Mosul. Iraqi Interior Minister Mohammed Salem al-Ghabban said Monday that the campaign had halted temporarily to allow civilians in the city to evacuate and to enable troops to clear roadside bombs planted by the extremists.

Sunni grievances have grown since the U.S.-led invasion toppled Saddam in 2003 and handed power to the long-oppressed Shiite majority. Under former Prime Minister Nouri al-Maliki, the Sunnis complained of discrimination by the Shiites, saying they were neglected or exploited through corruption.

Over the course of the eight-year U.S. occupation, militants from al-Qaida tried to capitalize on the Sunni grievances, and those extremists soon morphed into the organization that now calls itself the Islamic State group.

Many of the Sunni tribesmen eventually fought back. Members formed ad hoc militias known as Sahwa, or Awakening Councils. The movement was supported by Washington, which sought to empower locals entrenched in some of the insurgent group's biggest strongholds. But commitment soon waned from the U.S. and Iraqi governments, and the movement quickly lost momentum.

The extremists of Islamic State rampaged across northern and western Iraq last summer, looking to capitalize on the grievances and claimed to offer an alternative to the oppression felt by many in the Sunni heartland. Those Sunni tribesmen who remained opposed to group's ideology — like al-Jabouri — were targeted.

With the support of the U.S.-led coalition fighting the extremists, the government of Prime Minister Haider al-Abadi has tried to empower Sunnis by establishing a national guard — a force some see as reminiscent of the failed Sahwa movement.

In vast Anbar province in western Iraq, about 5,000 tribesmen are on board with government efforts to fight the IS group. The tribesmen receive arms and financial compensation.

But many Sunnis have not been won over, and with tribes often numbering 30,000 to 40,000 people, the effort has a long way to go.

"I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities," al-Abadi said last week, indirectly calling out to members of the outlawed Baath party, loyalists to Saddam, and other Sunnis who took up arms with the Islamic State against Baghdad's Shiite-led government.

But the uneasy alliance of men like al-Jabouri with the powerful Shiite militias is a reminder that conflicts in Iraq are never black and white. The militias are accused of brutal revenge attacks against Sunni civilians, raising concerns that these battlefield partnerships might be short-lived and could further aggravate the country's longstanding sectarian rifts.

Earlier this month, New York-based Human Rights Watch urged the Iraqi government to protect civilians in Tikrit and allow them to flee combat zones. Its statement noted "numerous atrocities" against Sunni civilians by pro-government militias and security forces.

Ahead of the operation, the Shiite prime minister, al-Abadi, called on Sunni tribal fighters to abandon the Islamic State group, offering what he described as "the last chance" and promising them a pardon.

But Mohammed al-Jabouri, another fighter with the Sunni brigade, said he doesn't worry that his Shiite comrades might eventually turn on him.

"It's Daesh who killed our sons and our relatives and demolished our homes, so I am fighting with our Shiite brothers to avenge them," he said. "I have no concerns that the Shiites will harm us. They are risking their lives in the same way we are risking ours. They are our brothers."

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Associated Press writer Vivian Salama contributed to this report.