As American forces open another front of battle in Iraq, they find themselves on the same side as an array of armed groups that not only consider the United States an enemy but also accuse it of actively supporting Islamic State militants.

Since the U.S.-led coalition planes launched their first airstrikes in the Islamic State-held city of Tikrit on Wednesday night, threats and accusations from ­Shiite militias who were leading the battle there have grown. Several of the Iranian-backed groups accused coalition aircraft of bombing a headquarters for pro-government fighters in the city on Friday, promising retribution.

The U.S. Embassy in Baghdad released a statement Friday rebutting the claim, saying there were no coalition strikes in the vicinity at the time, and the Iraqi government also said no such attack took place.

The claim was the latest in a long string of accusations leveled at the United States since its first airstrikes against the Islamic State in August. Rumors of coalition planes dropping weapons supplies to Islamic State militants and attacking pro-government fighters are now widely held beliefs in a country where conspiracy theories are rife.

However baseless, the accusations highlight the United States’ precarious position, which is ­unwelcome to many Iranian-
supported Shiite groups fighting on the ground against Sunni ­Islamic State militants. Analysts say the groups’ intensifying threats are likely an attempt to dampen the will of the international coalition as the United States and Iran vie for influence in the battle.

The United States’ intervention in the Tikrit offensive, which came after the Shiite militias’ ­efforts stalled, has left the Iranian backed-groups feeling undermined, according to their members.

“We will respond with force while they are within our firing range,” Shibil al-Zaidi, a leader of the Kitaeb Imam Ali militia, said in a statement about the alleged U.S. strike on Friday. “We have the ability to face these American attacks.”

Most Shiite militia leaders and some commanders in Iraq’s so-called popular mobilization units, a grouping of militiamen and Shiite volunteers, have refused to fight in the battle for Tikrit until U.S. planes leave. But they also say they have refused to leave their positions, and some have threatened to shoot down American planes.

Kitaeb Hezbollah, a Shiite militia that is classified as a terrorist organization by the United States and carried out major attacks on U.S. targets during the Iraq war, was the first to make that threat on Wednesday.

Earlier this month, the group showed off its surface-to-air missiles in a video in what Phillip Smyth, a researcher at the University of Maryland specializing in Shiite militant groups, said was probably part of a calculated attempt to intimidate.

Conspiracy theories run deep

But that enmity for the United States circulates beyond the militias that once fought U.S. soldiers, surfacing also in parliamentary debates and Iraqi media reports and even at the highest ranks of the national armed forces that the United States is aiding.

“Everybody knows that the Americans are dropping supplies to Daesh,” said Brig. Gen. Abed al-Maliki, a senior Iraqi army commander based in the city of Samarra, about 80 miles north of Baghdad, using another term for the Islamic State.

What’s more, he said, during some of the fiercest fighting around Samarra last year, U.S. Special Operations forces dropped behind enemy lines to assist Islamic State militants.

“They came in with parachutes, and they were helping to bomb the city,” he said.

U.S. airstrikes against the Islamic State, he contended, are probably just a cover for efforts to support the group.

“It’s just a show,” he said, sitting in the city’s army command headquarters. “If the Americans want to finish something, they will finish it. If they wanted to liberate Iraq, they could.”

A U.S. defense official, who spoke on the condition of anonymity because he was not authorized to discuss the matter, called the notion of the United States assisting the Islamic State “ludicrous,” given that members of the U.S. military were risking their lives to fight the same group.

Speaking about hostility from some militia groups, the official said that “these are some of the militias that fought us during the Iraq war, when we were there earlier . . . so we take what people are saying seriously.

“But for right now we believe that our service members and our aircraft are safe, and we’ll continue to take the fight to ISIL,” he added, using an acronym for the Islamic State.

When such accusations appear in the Iraqi media, they are normally accompanied by an image from an Islamic State video from Kobane in Syria last year, showing the militants displaying a load of weapons accidently dropped from a U.S. plane — an incident the United States acknowledged.

Visiting U.S. officials are left to fend off questions about whether they support the group. The topic was the first to be broached in questions when Gen. John Allen, special envoy for the coalition to counter the Islamic State, met with Iraqi journalists in January.

“The story, I think, is that we’re supplying [the Islamic State],” he said. “And that, in fact, is not correct.”

The theories are stoked by U.S. involvement in the wider region, where Sunni states such as Saudi Arabia are battling for influence against Shiite Iran. While the United States has backed the same side as Saudi Arabia in conflicts in Syria and Yemen, in Iraq it finds itself on the other side of the battle.

A wildly popular trailer for an Iraqi TV program launched last year that mocked the Islamic State played off that speculation. It showed Islamic State leader Abu Bakr al-Baghdadi hatching out of an egg after a marriage between characters representing Israel and America.

Mustafa Salim in Baghdad and Missy Ryan in Washington contributed to this report.

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