BAGHDAD, Iraq — The sectarian cleansing that has turned suburban districts of Baghdad into nightly war zones is reaching new heights as areas where Sunni and Shiite Muslims lived side by side for centuries now house only one community.

New battle lines have been drawn across the capital amid a surge in violence, and members of the rival sects are abandoning their homes.

The two-lane highway that divides the Sunni district of Ghazaliyah from Shiite Shuala in northwestern Baghdad is one such battle line.

At first glance yesterday, it looked like any other street: Cars passed, a man pushed a cart piled high with folded shirts, young men chatted. But all is far from normal. The shops that in Saddam Hussein’s time opened long into the night were mostly shuttered. The curtains in the houses were closed. No one crossed the street.

Once darkness falls, each side erects barriers, and locals say there is often fighting. Ali, a Sunni in Ghazaliyah, said he and his neighbors recently saw gunmen approaching.

“They started firing at our houses,” he said. “They didn’t expect a very quick response, but we gave them one. … We surrounded them. They were in a trap, and gunfire on them was from everywhere. We killed a lot of them. After defeating them, it was our turn to attack. We followed them, and we saw them entering a mosque, which we shot with two rocket-propelled grenades. And then we returned home.”

It is a scenario being repeated across Baghdad. In its southern districts, 95% of Shiites have been forced out of the Sunni stronghold of Dora.

Sunnis have mostly gone from the Shiite district of Zaphrannia to its west.

Eight Sunni families left one street there in 24 hours after a Sunni neighbor was killed in his home. A similar “cleansing” of Sunnis occurred in Mashtel, while in nearby New Baghdad both Sunnis and Shiites fight nightly battles.

The fighting worsened after the bombing in February of the Golden Mosque in Samarra, a Shiite shrine, which led to a rampage by Shiites and retaliation from Sunnis.

There have been reports that the government is contemplating formalizing these divisions by splitting the city along the Tigris into the mostly Sunni west and the mostly Shiite east. Yet reality is not as neat. All the districts mentioned above are west of the river and include hundreds of thousands of Shiites.

Rather than a simple west-east split, Baghdad is a collage of bordering and increasingly hostile sectarian neighborhoods.

In some parts, there are still signs that Sunnis and Shiites are willing to protect each other from outside threats. One local Shiite in the central commercial district of Karada recently publicly warned against attacks on Sunnis.

“If anyone touches them, they touch us,” he said.

But in a city where fear rules, such declarations of unity are increasingly the exception as most look to their own for protection.