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Iranian weapons are turning up in the Islamic Republic’s former arch foe Iraq, a development one State Department official told FoxNews.com warrants an immediate investigation.

Photos of Iraqi soldiers bearing Iranian-made rifles and scopes as they battle Al Qaeda-linked Islamic extremists in western Iraq have surfaced on the Internet. The images have raised suspicions that Iraq, which already gets large amounts of arms from the U.S., is buying the weapons from the neighboring nation with whom it fought a bloody and bitter war throughout the 1980s. Such transfers would violate United Nations sanctions, and raise concerns about Iran's ever-increasing reach throughout the Middle East.

“We continue to reiterate with the government of Iraq that any transfer or sale of arms from Iran would be in violation of UNSC resolutions and support a full investigation into the matter,” a State Department official told FoxNews.com.

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The apparent cooperation between the two nations springs from a tangled web of Middle Eastern alliances: Iran supports Syrian dictator Bashir Assad, who, like Iraq, is being attacked by the extremist group known as the Islamic State of Iraq and the Levant (ISIL). So by arming Iraqis, Iran puts additional pressure on the extremist group and helps Assad stay in power.

Officially, both Iraq and Iran deny the arms deal. But documents obtained by Reuters indicate that a deal was struck, and photos discovered by the military blog War is Boring show that Iranian sniper rifles are now turning up on the battlefield.

State Department officials said that the U.S. is doing everything it can reasonably do to support Iraq in its fight against ISIL extremists – which should lessen the need for Iranian weapons.

“We view the government of Iraq as a partner in the fight against terrorism and are committed to supporting it in its fight against ISIL. We have provided the Iraqi military and security forces with more than $15 billion in equipment, services and training… including over $200 million in expedited security assistance we have delivered since the beginning of this year,” the official told FoxNews.com.

The U.S. arms shipments to Iraq span a vast array of weaponry, including 300 Hellfire missiles, thousands of rounds of tank ammunition, helicopter-fired rockets, machine guns, grenades, flares, sniper rifles, M16s and M4 rifles. This week, the Pentagon announced $1 billion in additional military aid, including trainer airplanes and surveillance equipment.

But despite U.S. help, Iranian weapons are also appearing on Iraqi battlefields, and security experts say that the two countries working together is worrisome.

“The risk of what would happen if Iran came to dominate Iraq is huge, and Iraq is absolutely critical to the stability of the Gulf and its global oil exports,” Anthony Cordesman, of the Center for Strategic and International Studies, told FoxNews.com.

It could be the precursor to Iran coming to dominate all of the Middle East, he added. “You could end up with an Iran capable of playing a dominant role in Iraq, Syria and Lebanon.”

The author of the piece, Maxim Lott, can be reached on Twitter at @maximlott or at Maxim.Lott@FoxNews.com