Anyone who tries to slip out of the Shiite villages of Zahraa and Nubl is risking his life. Sunni rebel snipers stand ready to gun down anyone who dares. Roads are blocked with barricades and checkpoints.

For more than three months, Syria's rebels have imposed a smothering siege on the villages, home to around 35,000 people, maintaining they are a den of pro-regime gunmen responsible for killing and kidnapping Sunnis from nearby towns.

The bitterness and reprisals between neighbors illustrate how the civil war has torn apart the longtime coexistence among ethnic and religious groups in Syria. And it points to the perils of sectarian divisions that lie ahead for the nation of 21 million as the war worsens.

Zahraa and Nubl make up a small pocket of Shiites, mostly regime loyalists, in this overwhelmingly Sunni region in the northern countryside of Aleppo province. The siege has its roots in months of tensions since the Sunni-led revolt against President Bashar Assad began in March 2011. Sunnis in the area say pro-regime gunmen, known as Shabiha, operated from the two villages, attacking nearby towns as they rose up against Assad.

The violence fueled a cycle of tit-for-tat killings and kidnappings and tore apart the social fabric between the sects.

Then in July, rebels overwhelmed most of Aleppo province, driving out government forces and taking control of the region's towns and villages. The tables were turned: Many Assad loyalists fled to Zahraa and Nubl for refuge, and the rebels clamped down their siege, seeking revenge.

Perhaps more than anyone, Bashar al-Hajji feels the impact of the rift. A native of Beyanon, a Sunni village of 5,000 across the main north-south highway from Zahraa and Nubl, he's the only Sunni in town who's married to a Shiite. His wife of five years is from Zahraa.

"I am caught between the two sides," said al-Hajji, a 28-year-old mechanic who is not just Sunni but follows the sect's most conservative school, Salafism.

His wife's family shuns him and is persistently telling her to leave him and come home to Zahraa.

"They know that I am a Salafi, and they think it is permitted for me to kill Shiites," said al-Hajji, sitting in the yard of the family's home in Beyanon.

"Well, if this is the case, I don't have to go far to kill one," he said, nodding toward his house.

Al-Hajji limps from a gunshot wound he suffered in February, when a gang of Shiites from Zahraa beat and kidnapped him.

"Only when my family and others kidnapped about 20 of their own and threatened to kill them was I released," he said. He showed a photograph taken after his release, his face bruised and a deep cut across his cheek.

He pointed to a neighborhood of Zahraa in the distance and said it had been home to pro-regime snipers and machine gun nests that shot at "everything that moves" in Beyanon.

"They killed and wounded so many of us, we had to block the road," he said.

A friend, Khaled Mohammed Saraj, a 29-year-old carpenter, was kidnapped by Shiites in July while driving near Zahraa at 6 a.m.

"They kept me in an underground room for six days," recounted the father of two. Three others were also held there, he said, though their captors did not abuse them. They were eventually released in a prisoner exchange.

Now dirt mounds — and in some cases slabs of limestone — block the roads leading into Zahraa and Nubl. Rebels have set up checkpoints, and snipers are positioned in empty buildings. Zahraa has its own snipers, on a hill overlooking the road, and they open fire on anyone they see trying to get into the village, fearful of rebel attacks. The barricades were the closest The Associated Press was able to get to the two villages.

In the distance, Syria's national flag — now the symbol of Assad's regime — was flying from a water tower in Zahraa, unlike the rebel flags raised in Sunni towns. The Shiites in Zahraa and Nubl are coping with the siege by relying on supplies from a friendly Kurdish village on the other side. But they cannot venture further than that for fear of being killed or kidnapped. Government helicopters land there twice a day, bringing in supplies.

For Beyanon and a cluster of nearby Sunni villages — Hayan, Retan, Haritan and Mayer — the feud means they can no longer go into the two larger Shiite villages, or have access to their restaurants, better-stocked grocery stores, mechanics and doctors. The two villages have the area's only high school, along with a vocational institute.

"Our villages are poor. Zahraa and Nubl are rich and have so much that we don't have," said al-Hajji, one of six siblings born to a retired military man with 30 years of service in the air force as an aircraft mechanic.

The tears in the social fabric are nationwide. Syria's multiple sects, religions and ethnicities long coexisted — not always completely in tune, but usually more harmoniously than in neighboring Lebanon. The country is predominantly Sunni Muslim, while Shiites make up a tiny proportion, less than 5 percent, though exact numbers are not known.

The most serious split is between Sunnis and Alawites, an offshoot sect of Shiism that makes up about 15 percent of the population and dominates Assad's regime. Assad and his family are Alawites and elevated their community to top military and government positions.

Like the Alawites, Syria's small Shiite community has largely sided with Assad's regime, as has the small Christian minority, which fears the rise of Sunni fundamentalists if Assad falls.

"Syria has sustained sectarian scars that will simply take a long time to heal," said Loai Hussein, an opposition political activist. "There has been a great deal of polarization, some of which is encouraged by neighboring countries."

There have been countless cases of tit-for-tat slayings between Sunnis and Alawites or Shiites across much of the country. In the main cities, mixed Alawite-Sunni neighborhoods are fast disappearing, as residents of one sect are driven out by the other or flee to areas where their community is the majority.

In neighboring Idlib province, which has been the scene of heavy fighting, a Shiite leader from the village of Kifaraya said his son-in-law was kidnapped seven months ago by Sunnis.

"I hope it is all part of the war and that it will go away when it is over and we can live together again in peace," said Abu Abdullah Hassaneh. "I am not pointing an accusing finger at anyone. It is haram (religiously prohibited) to do so."

In a wider context, Syria's sectarian fault lines reflect a divide in the region. Shiite powerhouses Iran and Hezbollah have backed Syria, while Sunni Egypt, Saudi Arabia and their smaller Arab allies have aided the rebels and called on Assad to step down. Syria's rebels accuse Hezbollah guerrillas and Iraqi Shiite hard-liners of fighting alongside Assad's forces, though they offer no proof.

While Syrian opposition politicians in exile speak of inclusion and equality in a post-Assad Syria, the Sunni rebels on the ground bitterly talk of the need to exact retribution against Alawites they say victimized them for decades.

Al-Hajji and fighters of the rebel Free Syrian Army insist they will lift the siege on Zahraa and Nubl if the government forces and Shabiha surrender.

But their desire for vengeance shows through. They say the Shiite villagers are doing more than providing sanctuary for the regime loyalists and accuse them of taking part in kidnappings and sniping at Sunnis.

Al-Hajji's wife's brother was wounded in a recent firefight with Sunnis, he said, but her family said nothing of his injury. They are also evasive when she asks them about conditions in the two villages.

Al-Hajji's 62-year-old father, Abdou al-Hajji, adopts a conciliatory tone when speaking about the divides. He notes how Sunnis "embraced" Shiites who fled Lebanon during Hezbollah's war with Israel in 2006.

"We have long lived in peace alongside the Shiites," the elder al-Hajji said. "We shared joy in weddings and grief in funerals."