Sunni Iraqis who fled Fallujah with their families carry blankets and mattresses distributed by the International Organization for Migration in Karbala, the holy Shiite city which is now hosting Sunnis. (AHMAD AL-RUBAYE/AFP/Getty Images)

The plush accommodation halls on the outskirts of this southern Iraqi city, normally reserved for visiting Shiite pilgrims, now teem with displaced Sunnis fleeing violence in the western province of Anbar.

There and elsewhere, sectarian tensions are brewing as Iraq spirals into the worst cycle of violence it has experienced in years. But here, in one of the holiest cities for Shiite Muslims, Sunni children play on brightly painted swings as families gather in the waning winter light beside clipped magnolia-lined lawns.

The scenes are an effort by Shiite religious authorities to portray a picture of harmony as sectarian violence grows. Al-Qaeda’s local franchise, the Islamic State of Iraq and Syria, or ISIS, is building strength in Anbar amid a Sunni-majority population that is growing increasingly disillusioned with the Shiite-led federal government.

Along with a murky mix of allies, ISIS has become deeply embedded in the Anbar city of Fallujah, which has slipped out of state control. Militants, as well as anti-government tribesmen, are tightening their grip, leaving many refugees to question whether they will be able to return home — or whether they even want to.

“They will have to drag me screaming from this place,” said one refu­gee, a 57-year-old woman who spoke on the condition of anonymity to protect relatives who have remained in Fallujah. “We came with nothing, but nothing will take me back there. The armed men control it. I wish to prophet Muhammad that someone would drop a bomb on it.”

The battle between Islam's two major branches began centuries ago, but it's still affecting Iraq's path to a stable democracy now. The Post's senior national security correspondent Karen DeYoung explains. (Davin Coburn/The Washington Post)

In Karbala, Mudafa Mohammed Shaban, the manager of the pilgrims’ complex, describes the city’s hosting of displaced Sunnis as a “real message” — proof that there is no sectarianism in Iraq.

But few Iraqis would agree.

The 72 families in the compound, just a fraction of the more than 14,000 families that the United Nations says have been displaced from Anbar, undergo heavy vetting before they are allowed to stay. Names are checked against blacklists kept by state security agencies. New checkpoints search cars crossing the border from Anbar into Karbala province, and the road is closed at night.

There is an acute awareness nationwide that even a minor incident could spark sectarian bloodletting of the kind seen in 2006 and 2007, when Sunni and Shiite death squads wreaked havoc across the country. Iraq’s notorious Shiite militias have largely refrained from avenging the frequent bombings in their neighborhoods in Baghdad, but an attack on a sensitive religious site, such as one of Karbala’s shrines, could change that.

It is the second or third time that many Fallujah residents have been forced to flee. Several interviewed here said a majority of city’s population has left.

“Fallujah is a city of ghosts,” said one 32-year-old man, who spoke on the condition of anonymity out of concern for his safety.

Fallujah and surrounding Anbar have long been plagued by violence. One-third of the U.S. soldiers who died in the Iraq war were killed in the province. Memories of the two bloody assaults on Fallujah in 2004, when U.S. forces tried to oust militants, are still raw among residents.

This time, the Iraqi government says it has ruled out sending in the army. Troops remain stationed outside Fallujah but clash frequently with militants on the highway, which military vehicles traverse on their way to Ramadi, the capital of Anbar.

The government says it will leave it to the tribes to purge al-Qaeda and its affiliates from the city, in an echo of the U.S.-backed “Awakening” tribal movement that helped restore calm to Anbar during the latter years of the Iraq war. The decision is a nod to Iraq’s sectarian reality: An assault by what Anbar residents view as a Shiite army could further inflame the situation.

But at the pilgrims’ complex in Karbala, many of the displaced said they doubt local tribesmen have the power to combat ISIS and fear the government might lose control of Fallujah for good.

“The tribes don’t have the weapons,” said Abu Duaah, 47, who fled the city this month. “Al-Qaeda’s weapons are stronger.”

The government says it is sending arms and money to Sunni tribes in Anbar. But it is reluctant to fully back tribal leaders, many of whom were involved in a major anti-government protest movement in Anbar last year.

For those who have fled, the situation remains a waiting game. At the Karbala compound, the 57-year-old woman from Fallujah said this is the third time that she has fled violence and insisted that it will be the last. She left with nine members of her extended family, who crammed into one car and left most of their possessions behind.

She said she had heard on television that the displaced from Anbar were being offered a place to stay in Karbala.

She said, sitting cross-legged on the floor, that she realizes she is one of the lucky few. Here, the Sunnis from Anbar are taken on bus tours of the city to see Shiite religious sites, offered free medical services at a private hospital and fed three meals a day, all paid for by Shiite religious authorities.

Still, she said it does little to distract from the deep sectarian divisions.

“We’ve had great hospitality here, but, really, the sweetness of old Iraq has gone,” the woman said, welling up.

About 45 miles southwest, in Ain Tamr, a city near the border with Anbar, about 180 displaced families are being put up by local families. As in Karbala, officials in Ain Tamr are eager to emphasize that the hospitality is a sign that most Iraqis stay above the sectarian fray.

But when asked how many of the refugees in the city are Sunni, and not from Anbar’s Shiite minority, local official Raed al-Mashhadani refused to answer, calling it an “embarrassing question.” He later acknowledged that those being hosted are mostly Shiites who complain that their lives are increasingly untenable in Anbar.

One of them is Haifa Khodair Abbas, 58, who said she would not go back to Fallujah, which had only a smattering of Shiite families.

“There is no place for Sunnis here. We just have a few families,” said one Ain Tamr resident. “If you want to find the Sunnis, you have to go that way to Rahaliya,” he says, signaling along the road west, over the border, to Anbar.