Iraq’s Shiite-led government has begun training Sunni tribal fighters here in the western province of Anbar, in an urgent U.S.-backed initiative to stem recent advances by Islamic State.

The setbacks in Anbar have exposed the need for trusted and equipped Sunni fighters to help turn momentum against Islamic State and dry up the extremists’ pool of potential recruits.

“When the government fails, people turn to Islamic State,” said Mohammad Abu Risha, a young tribal sheik from Anbar who commands 150 fighters. “The tribes don’t trust the government, and the government doesn’t trust the tribes.”

Mr. Abu Risha, speaking at an inaugural ceremony for the Anbar training program in Amriyat Fallujah, about 70 miles west of Baghdad, said the tribes had seen little support from Iraq’s prime minister until now.

More than 1,000 recruits, mostly from the Albu Issa tribe of Amriyat Fallujah, took marching orders on Friday as local officials cheered them on in speeches. Elsewhere in the province at the Habbaniya air base, bare buildings are being outfitted to host training there too.

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