By Phil Stewart

WASHINGTON (Reuters) - The United States plans to buy arms for Sunni tribesmen in Iraq including AK-47s, rocket-propelled grenades and mortar rounds to help bolster the battle against Islamic State militants in Anbar province, according to a Pentagon document prepared for Congress.

The plan to spend $24.1 million represents a small fraction of the larger, $1.6 billion spending request to Congress focusing on training and arming Iraqi and Kurdish forces.

But the document underscored the importance the Pentagon places on the Sunni tribesmen to its overall strategy to diminish Islamic State, and cautioned Congress about the consequences of failing to assist them.

"Not arming tribal fighters will continue to leave anti-ISIL tribes reluctant to actively counter ISIL," the document said, using another acronym for the group which has seized control of large parts of Syrian and Iraq and is gaining territory in Anbar despite three months of U.S.-led air strikes.

A U.S. official said on Saturday that the document was posted this week. Click http://1.usa.gov/11nsTuN to read it.

It said all U.S. support was directed "with, by and through" Iraq's government, suggesting any weapons would be supplied through Baghdad, in line with existing policy.

It noted Iraqi security forces were not "not particularly welcome in Anbar and other majority Sunni areas," citing their poor combat performance and sectarian divisions.

Iraq's army has been burdened by a legacy of sectarianism in Anbar, whose dominant Sunni population resented former Prime Minister Nuri al-Maliki's Shi'ite majority government and were incensed when he ordered troops to clear a protest camp in Ramadi in December 2013.

The ensuing Sunni tribal revolt prompted the entrance of Islamic State into Falluja and Ramadi, where U.S. troops had met fierce resistance from Sunni insurgents including al Qaeda during their occupation of Iraq after the 2003 invasion that overthrew Saddam Hussein.

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The United States, which has deployed a small number of military advisors to Anbar province, hopes the Sunni tribesmen can later form part of a more formal Iraqi National Guard.

The Pentagon document also detailed $1.24 billion to be spent on Iraqi forces and $354.8 million on Kurdish troops.

"While the trend on the battlefield has been promising in stemming ISIL gains, Iraq lacks the training expertise and equipment to field the forces needed to liberate territory," the document said.

(Reporting by Phil Stewart; Editing by Grant McCool)