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BAGHDAD -- In a sign of Iran's deepening involvement in the Iraqi crisis, the commander of Tehran's elite Quds Force is helping Iraq's military and Shiite militias gear up to fight the Sunni insurgents advancing across the country, officials said Monday.

Washington signalled a new willingness work with Iran to help the Iraqi government stave off the insurgency after years of trying to limit Tehran's influence in Baghdad -- a dramatic shift that would have been unthinkable a few weeks ago.

The White House also is considering sending a small number of U.S. special forces to Iraq to help the government slow the insurgency, U.S. officials said.

The insurgents seized the strategic city of Tal Afar near the Syrian border Monday, part of its goal of linking areas under its control on both sides of the Iraq-Syria frontier. West of Baghdad, an army helicopter was shot down during clashes near the city of Fallujah, killing the two-man crew, security officials said.

The Quds Force commander, Iranian Gen. Ghasem Soleimani, has been consulting in Iraq on how to roll back the al Qaeda-breakaway group, known as the Islamic State of Iraq and the Levant, according to Iraqi security officials.

Soleimani's presence in Iraq is likely to fuel longtime Sunni suspicions about the Shiite-led government's close ties with Tehran.

The security officials said the U.S. government was notified before Soleimani's visit.

Soleimani has been inspecting Iraqi defences and reviewing plans with top commanders and Iranian-backed Iraqi Shiite militias, the officials said. He has set up an operations room to co-ordinate the militias and visited the holy cities of Najaf and Karbala south of Baghdad, home to the most revered Shiite shrines, and areas west of Baghdad where government forces have faced off with Islamic militants for months.

The Islamic State has vowed to march to Baghdad, Karbala and Najaf in the worst threat to Iraq's stability since U.S. troops left in 2011. A call to arms Friday from Iraq's top Shiite cleric, Iranian-born Grand Ayatollah Ali al-Sistani, gave prominence to the need to defend the holy shrines.

Soleimani's visit adds significantly to the sectarian slant of the mobilization by the government of Prime Minister Nouri al-Maliki. Armed Shiite militiamen have been parading on the streets and volunteers joining the security forces are chanting Shiite religious slogans.

Al-Maliki rejects charges of sectarianism and points to recruiting efforts by some Sunni clerics, but there is no evidence of Sunnis joining the fight against the Islamic State in significant numbers, if at all.

The legitimacy accorded by his government to the Shiite militias poses a risk of Iraq sliding back into the deadly sectarian bloodshed of 2006 and 2007.

Such tensions were rising months before the Islamic State's lightning incursion of last week, with thousands killed since late last year. Bombings killed Shiites and security forces as militants took hold of vast territory and at least one city in the mainly Sunni Anbar province west of Baghdad.

Soleimani is one of the most powerful figures in Iran's security establishment, and his Quds Force is a secretive branch of Iran's Revolutionary Guard involved in external operations. In the mid-2000s, it organized Shiite militias in a campaign against U.S. troops in Iraq, according to American officials. More recently, it has been involved in helping Syrian President Bashar Assad in his fight against Sunni rebels.

His visit and the empowerment of the Shiite militias that his Quds Force trains and arms means Iran could take a role in Iraq similar to the one it plays in Syria. The Quds Force -- along with Iraqi and Lebanese Shiite fighters -- has been crucial to the survival of Assad, himself a member of a sect that is an offshoot of Shiite Islam.

U.S. Secretary of State John Kerry said in an interview with Yahoo! News that Washington is "open to discussions" with Tehran if the Iranians can help end the violence and restore confidence in the Iraqi government.

U.S. officials said earlier there is a possibility that a senior American diplomat may discuss Iraq with an Iranian delegation at nuclear talks in Vienna.

While President Barack Obama has explicitly ruled out putting U.S. troops into direct combat in Iraq, the plan under consideration suggests he would be willing to send Americans into a collapsing security situation for training and other purposes.

Three U.S. officials familiar with ongoing discussions said the potential of sending in special forces is high on a list of military options being considered.

It's not clear how quickly the special forces could arrive in Iraq or whether they would stay in Baghdad or be sent north.

The capture of the city of Tal Afar was a key prize for the militants because it sits on a main highway between Syria and Mosul, Iraq's second-largest city, which the Islamic State seized last week.

Iraqi military officials said about 500 elite troops and volunteers were flown Monday to Tal Afar and preparing to try to retake the city.

Tal Afar, with a population of about 200,000, is located 420 kilometres (260 miles) northwest of Baghdad. Its residents are mostly ethnic Shiite and Sunni Turkomen, raising fears of atrocities by Islamic State fighters, who brand Shiites as heretics.

Over the weekend, the group posted graphic photos purporting to show its fighters killing scores of Iraqi soldiers captured when it overran other areas.

Tal Afar Mayor Abdulal Abdoul said the city was taken just before dawn. One resident, Hadeer al-Abadi, said militants in pickup trucks mounted with machine-guns and flying black jihadi banners roamed the streets as gunfire rang out.

The local security force fled before dawn, and local tribesman who continued to fight later surrendered to the militants, al-Abadi said as he prepared to leave town with his family.

Another resident, Haidar al-Taie, said a warplane dropped barrels packed with explosives on militant positions inside the city Monday morning, and many Shiite families had left the town shortly after fighting broke out a day earlier.

"Residents are gripped by fear and most of them have already left the town for areas held by Kurdish security forces," al-Abadi said. The city is just south of the self-rule Kurdish region and many residents were fleeing to the relatively safe territory, joining refugees from Mosul and other areas that have been captured by the militants.

Some 3,000 others from Tal Afar fled west to the neighbouring town of Sinjar.

Since the 2003 U.S.-led invasion, Tal Afar was often hit by car bombings and other attacks by Sunni militants targeting its Turkomen minority.

At one point, after a major American offensive in 2006 to drive out insurgents, then-President George W. Bush declared Tal Afar a success story that shows "the outlines of the Iraq that we and the Iraqi people have been fighting for. ... A free and secure people are getting back on their feet."

Farther south, the ISIL militants battled government troops at Romanah, a village near another main border crossing to Syria in Anbar province, according to a security official in Baghdad. The official spoke on condition of anonymity because he was not authorized to talk to the media.

The Islamic State already controls territory in Syria in several regions next to the Iraq border. Its fighters move relatively freely across the porous, unprotected desert border, along with money, weapons and equipment. Seizing an actual border crossing, however, would be a major symbolic gain for the group.

Also Monday, militants ambushed a vehicle carrying off-duty soldiers to Samarra, a city north of Baghdad that is home to a much-revered Shiite shrine. Six soldiers were killed and four wounded, a government official said.

Security has been tightened around Baghdad, particularly on its northern and western edges, and food prices have dramatically gone up because of the transportation disruptions on the main road heading north from the capital.

Interior Ministry spokesman Brig. Gen. Saad Maan Ibrahim said Iraqi security forces killed 56 "terrorists" and wounded 21 just outside the capital in the last 24 hours. He made no mention of Tal Afar.

Security at the U.S. Embassy in Baghdad has been strengthened and some staff members sent elsewhere in Iraq and to neighbouring Jordan, the State Department said Sunday.

The State Department also cautioned U.S. citizens to avoid all but essential travel to Iraq. The warning said the Baghdad International Airport was "struck by mortar rounds and rockets" and the international airport in Mosul also has been targeted.

A senior Baghdad airport official, Saad al-Khafagi, denied the facility or surrounding areas have been hit. State-run Iraqiya TV also denied the attack, quoting the Ministry of Transport.

The United Nations said it has relocated 58 staff members from Baghdad, and may move additional personnel out of the capital due to security concerns.