IRBIL, Iraq - Iraqi security forces and Iranian-backed Shiite Muslim militias appeared Thursday to be on the verge of victory in a two-week effort to recapture Saddam Hussein's hometown from the Islamic State, which has held the symbolically important city since June.

But the seemingly certain triumph of a force with little Sunni Muslim participation in the center of Iraq's Sunni heartland has raised another troubling issue: the extension of Iran's influence in a country where the Shiite Muslim neighbor is already the most significant outside player.

The forces that appeared Thursday to have cornered the last Islamic State fighters in central Tikrit are dominated by Iranian military advisers. The Iraqi Shiite militias are all Iranian trained. And the offensive itself is being directed on the ground by Iran's most influential general, Qassem Suleimani, who has been a thorn in American efforts to pacify Iraq since the early days of the U.S. occupation.

To add to American unease, there are credible reports that Iranian troops and fighters from Lebanon's Hezbollah movement are participating in the Tikrit operation, and other reports that the Shiite militias and even U.S.-trained Iraqi troops have engaged in retaliatory attacks against Sunni residents. Those reports have convinced many Sunnis that the long-frayed relationship between Iraq's Sunni and Shiite sects is now completely broken.

"It's a Persian-led invasion of the Sunni triangle," said one prominent leader of a Sunni tribe who has fled both the central government and the Islamic State for the safety of the Kurdish capital of Irbil. "We see Iranian troops and generals leading the fighting and the only Iraqi army units - which once represented all Iraqis - now only represent the Shiite parties and their Iranian leadership."

He asked not to be identified because of fears he could be targeted by both sides of the increasingly bitter conflict.

"Look around Baghdad now and what do you see?" he asked rhetorically. "Posters dedicated to militia leaders, Iranian generals and even Khamenei," referring to Iran's supreme leader, Ayatollah Ali Khamenei.

"Daash is a poison to all Muslims, but the Persians have become a cancer to Iraq," he said, using the Arabic acronym for the Islamic State group. Then, referring to the eight-year-long war that Iraq under Hussein fought against Iran in the 1980s, he summed up: "What they could not do in the 1980s they have done now with American help, which is enslave Iraq."

The tribal leader pointed out that tens of thousands of Sunnis have offered to help the central government in Baghdad fight the Islamic State, but that the government of Prime Minister Haider al-Abadi, a Shiite, has rejected requests for arms from the Sunni tribes.

Even the populist Shiite cleric Muqtada al-Sadr, whose Mahdi Army battled American troops throughout the U.S. occupation, has refused to allow his militia to join the Tikrit operation because of what he claims is a constant pattern of innocent Sunnis being murdered or abused by other Shiite militias and the security forces.