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Iran is coming to the aid of its historic nemesis, sending elite fighters to Iraq in the wake of a Sunni insurgency that has claimed two key northern cities and now threatens Baghdad, Fox News has learned.

Some 150 fighters from the Revolutionary Guards elite Quds force have already been dispatched by Tehran, and the division's powerful commander, Qassem Suleimani, met with Iraqi Prime Minister Nouri al-Maliki Thursday and pledged to send two notorious Iranian brigades to aid in the defense of Baghdad. That could amount to as many as 10,000 soldiers sent to fight the Sunni group known as Islamic State in Iraq and the Levant (ISIS).

Maliki is believed to be considering the offer, especially in light of reported decisions by the U.S. to reject his request for American airstrikes against the Al Qaeda-affiliated militants who have recently overrun Mosul and Tikrit and appear to be preparing for a march on the capital. The two brigades that Suleimani offered are Asaab Ahel Haq, a Shi’ite paramilitary unit, and the Shi’ite insurgency group Kata'ib Hezbollah.

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After U.S.-trained security forces dropped their weapons and fled their posts in Mosul, the regime in Baghdad has reason to fear for its survival, an intelligence official said.

“Baghdad is going to be overrun," he said. "The Green Zone is going down.”

Although Iran and Iraq were at war in the 1980s, both the Maliki regime and the rulers in Tehran are Shi'ite, and Iran does not want a fanatical jihadist takeover of its neighbor. Iran has positioned troops along its border with Iraq and has threatened to bomb opposition forces if they come within about 60 miles of Iran’s border, according to an Iranian army general.

News about the fall of these two cities, which caused about 500,000 to flee, worried Iran. Mosul is in the western Iraqi province of the Biblically-mentioned Nineveh, which shares a 300-mile border with Syria, where the Iranian government has been pulling the political and financial puppet strings to keep President Bashar al-Assad in power against the opposing rebels and militants.

In addition to protecting the strategic border to Syria, Iran’s government has interests in safeguarding holy shrines and sites in Najaf and Karbala, significant to the Shiite Islamic religion. Many Iranians make pilgrimage to these sites every year.

Predominantly Shiite Muslim Iran will combat the "violence and terrorism" of Sunni extremists who have launched an anti-government offensive in neighboring Iraq, Iran’s President Hassan Rouhani warned on Thursday.

“This is an extremist, terrorist group that is acting savagely," Rouhani said live on state television.

If Iraq's pleas for support are rebuffed by the U.S., it may have no choice but to turn to Iran, said experts.

“My sources tell me Maliki believes he is in a desperate situation and wants and needs our support," said retired four-star Gen. Jack Keane, former vice chief of staff of the U.S. Army. "If he doesn't get it in a way that will help him, he will certainly turn to Iran.”

Iran has more to offer than just the region's most powerful army, Keane said. Tehran could support Maliki with intelligence and advisors, too.

ISIS, a Sunni Islamic jihadi group, which is an offshoot of Al Qaeda, has gained control of geopolitically vital cities in both Syria and Iraq over the last year. It considers Shi'ite Muslims heretics that must be killed at the sword. Its goal is to cleanse Iraq from its Shiite influences.

ISIS spokesman Abu Muhammad Al-Adnani said in an audio released by intelligence sources that the group is planning to march toward Baghdad and other pivotal cities, including Karbala and Najaf.

"March to Baghdad al-Rashid, the Baghdad of the Caliphate. We have a score to settle...Be certain of the victory of Allah as long as you fear Him," Al-Adnani said in the recording.

As ISIS forces have stormed their way across northern Iraq, they have put into effect Sharia law on the citizens of Nineveh province, circulating a document on social media warning local leaders and religious sheikhs not to “work with (the Iraqi) government and be traitors.” The document also prohibits women from leaving the house unless absolutely necessary and for women to “dress decently and wear wide clothes.”

The document also bans drugs, alcohol, cigarettes in public and the possession of guns and non- ISIS flags.

ISIS terrorists in Iraq are allegedly made up of Tunisians and Yemenis, along with other “international fighters,” according to one Iraqi witness.

As the militants went from Mosul to Tikrit, they seized oil fields in Salahuddin province and looted the central bank and collected $420 million. They also took 48 Turkish citizens hostage as they seized the Turkish consulate in Mosul, which could bring another regional power down on them. Many eyes are on Turkey, a NATO ally, that has shown interest in northern Iraq for some time now for economic reasons and to support Iraq’s marginalized Kurdish minority.

Prime Minister Recep Tayyip Erdogan of Turkey held an emergency briefing with high ranking security officials and the Turkish foreign minister, Ahmet Davutoglu, who said, “No one should try to test the limits of Turkey’s strength.”

Fox News' James Rosen contributed to this report.